Skip to content
https://www.bl.uk

Performance

· 15 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
70
GRADE
C
FIX
5
REVIEW
2
PASS
6
INFO
2

Performance-focused optimization plan with CWV assessment, top offenders, and time estimates

Mobile Desktop
86
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
100
Accessibility Measures how accessible the page is for users with disabilities. Checks color contrast, ARIA labels, and semantic HTML.
100
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
92
SEO Measures basic SEO optimizations: meta tags, crawlability, link text, and mobile friendliness.

First Contentful Paint First Contentful Paint — how long until the browser renders the first piece of content. Under 1.8s is good.

413 ms

Largest Contentful Paint Largest Contentful Paint — how long until the largest visible element loads. Under 2.5s is good.

989 ms

Total Blocking Time Total Blocking Time — total time the main thread was blocked, preventing user input. Under 200ms is good.

232 ms

Cumulative Layout Shift Cumulative Layout Shift — measures visual stability. How much the page layout shifts during loading. Under 0.1 is good.

0.000

Speed Index Speed Index — how quickly content is visually displayed during load. Under 3.4s is good.

2.09 s

Time to Interactive Time to Interactive — how long until the page is fully interactive and responds to user input. Under 3.8s is good.

2.42 s

Page Load Progression

606 ms
1.2 s
1.8 s
2.4 s
3.0 s
3.6 s
4.2 s
4.8 s
Checks
15
6 PASS 2 REVIEW 5 FIX
F
Page Weight Budget
Action
4.4 MB transferred, 47 requests
FIX
4.4 MB transferred, 47 requests
Critical::
Page weighs 7.7 MB (4.4 MB transferred)
Warning::
JavaScript is 1.4 MB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 1.4 MB
Info::
47 HTTP requests
Info::
Estimated 0.91 g CO2 per page load
4.4 MB / 2.4 MB budget
4.4 MB transferred
47 requests
0.91 g CO2 per page load
Other 2.5 MiB 58%
JavaScript 1.4 MiB 32%
Images 334.9 KiB 8%
HTML 52.0 KiB 1%
Fonts 23.4 KiB 1%
CSS 20.3 KiB 0%
Other 1.0 KiB 0%
Other 629 B 0%
Other 609 B 0%
0 2.4 MB 4.9 MB
Over budget

Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.

Why this matters

JavaScript bundle is large — code-split routes and lazy-load off-screen components to defer.

Source: web.dev

D
Third-Party Impact
Action
78% third-party, 0 ms blocking
FIX
78% third-party, 0 ms blocking
Warning::
Third-party code accounts for 78% of page weight (3.4 MiB of 4.4 MiB)
Info::
Third-party blocking time is low (0 ms)
22%
78%
First-party Third-party
F
JS Bundles
Action
23 scripts, 734 KB unused
FIX
23 scripts, 734 KB unused
Warning::
https://www.bl.uk/_next/static/chunks/021g8z6d-ga-...: 214 KB unused (100%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=G-B8DB...: 69 KB unused (40%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-861...: 66 KB unused (46%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-164...: 65 KB unused (46%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-987...: 65 KB unused (48%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 734 KB
734 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js215 KB214 KB (100%)✓1st
https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1h1172 KB69 KB (40%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1h1144 KB66 KB (46%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1h1144 KB65 KB (46%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1h1137 KB65 KB (48%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD169 KB61 KB (36%)✓3rd
https://www.bl.uk/_next/static/chunks/13.otg-600jda.js69 KB61 KB (88%)✓1st
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js123 KB55 KB (45%)✓3rd
https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js29 KB29 KB (100%)✓1st
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js71 KB26 KB (37%)✓1st

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

Consider code splitting or tree shaking to reduce unused code.

Why this matters

This bundle has high unused code — code-split or tree-shake to ship only what executes.

Source: web.dev / Lighthouse coverage

F
Page Weight Inventory
Action
47 resources · 4.4 MB · 734 KB savings available
FIX
47 resources · 4.4 MB · 734 KB savings available
Warning::
Page weight 3.4 MB over 1MB target
Got: 4.4 MB Expected: ≤ 1 MB
Info::
~734 KB of savings available
Warning::
15 third-party resources (78% of weight)
Info::
2 resources over 200KB
47 resources · 4.4 MiB · 15 third-party 3.4 MiB over 1MB target
Media 58% · 2.5 MiB
JavaScript 32% · 1.4 MiB
Image 8% · 336.0 KiB
Document 1% · 52.0 KiB
Font 1% · 23.4 KiB
CSS 0% · 20.3 KiB
Other 0% · 1.2 KiB

~733.5 KiB of savings available

Remove unused JavaScript: 733.5 KiB
D
HTTP Caching
Action
no-store (uncacheable)
FIX
no-store (uncacheable)
Info::
Cache-Control: no-store prevents all caching
Intentional for dynamic content, but increases server load for every page view.
Got: private, no-cache, no-store, max-age=0, must-revalidate
Info::
No ETag or Last-Modified header
Conditional requests (304 Not Modified) are not possible without validators.
Cache-Control

private, no-cache, no-store, max-age=0, must-revalidate

DirectiveValueMeaning
private—Only browser cache, not CDN/proxy
no-cache—Cache but always revalidate first
no-store—Never cache this response
max-age0Cache for 0 seconds
must-revalidate—Must recheck with server after expiry
C
JS Execution Cost
Action
1194ms total JS execution
REVIEW
1194ms total JS execution
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-TSF...: 217ms CPU time
Info::
Unattributable: 181ms CPU time
Info::
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx9...: 161ms CPU time
Info::
https://www.bl.uk/: 140ms CPU time
Info::
https://www.googletagmanager.com/gtag/js?id=G-B8DB...: 133ms CPU time
Warning::
Third-party scripts: 893ms (75% of total)

Main Thread Breakdown

Script Evaluation 58% Other 16% Script Parsing & Compilation 15% Style & Layout 6%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD217ms168ms24ms3rd
Unattributable181ms3ms0ms3rd
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js161ms142ms9ms1st
https://www.bl.uk/140ms13ms12ms1st
https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1h1133ms110ms22ms3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js119ms100ms19ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1h193ms74ms19ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1h181ms64ms16ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1h168ms54ms13ms3rd
C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

A+
Text Compression
All text resources are compressed
PASS
All text resources are compressed
Info::
All text resources are compressed
All text resources are properly compressed.
A+
Image Optimization
7 images, 0 KB saveable
PASS
7 images, 0 KB saveable
Info::
All images are well-optimized
7 images 335 KB
0 oversized -0 KB
0 legacy format
0 missing dimensions CLS risk
A+
Font Loading
1 fonts (23 KB)
PASS
1 fonts (23 KB)
Info::
1 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

1

23 KB total

Render-blocking

0

of 1

Dominant font-display

swap

Most common across fonts

Font loading timeline
Transfer FOIT (block) FOUT (swap)
0 ms 500 ms 1.0 s FCP 413 ms LCP 989 mshttps://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2Hind_Variable-s.p.0238s... https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2 — 23.4 KiB — 106 ms — font-display: swap
Hind_Variable-s.p.0238s... woff2 swap
Size 23 KB
Load time 106 ms
Start 1.2 s
Risk FOUT — text flashes from fallback to web font
Optimization checklist
  • Preload critical fonts (priority=high)
  • Use woff2 format for all fonts
  • Set font-display to swap, optional, or fallback
  • Subset large fonts (≤100 KB each)
A+
Resource Caching
All resources properly cached
PASS
All resources properly cached
Info::
No caching issues found
All static resources have appropriate caching headers.
A+
Critical Rendering Path
No render-blocking resources
PASS
No render-blocking resources
Info::
No render-blocking resources detected
A+
Resource Hints
No optimization needed
PASS
No optimization needed
Info::
No resource hint issues
Network Waterfall
47 requests over 2528ms
INFO
0 ms 500 ms 1.0 s 1.5 s 2.0 s 2.5 s FCP 413 ms LCP 989 mshttps://www.bl.uk// https://www.bl.uk/ — 200 — 52.0 KiB — 1.2 s — h2https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2/_next/static/media/Hind_Variable-s... https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2 — 200 — 23.4 KiB — 106 ms — h2https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.css/_next/static/chunks/12v9u.4p6sjz2.... https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.css — 200 — 1.0 KiB — 67 ms — h2https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.css/_next/static/chunks/0cfu9oxbid~j~.... https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.css — 200 — 13.7 KiB — 105 ms — h2https://www.bl.uk/_next/static/chunks/082s_d0fh06im.css/_next/static/chunks/082s_d0fh06im.... https://www.bl.uk/_next/static/chunks/082s_d0fh06im.css — 200 — 3.5 KiB — 93 ms — h2https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css/_next/static/chunks/0b4yko77qod7i.... https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css — 200 — 937 B — 55 ms — h2https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.css/_next/static/chunks/129ls-rwdg-42.... https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.css — 200 — 1.2 KiB — 99 ms — h2https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GDwww.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD — 200 — 168.5 KiB — 47 ms — h2https://www.bl.uk/images/v5dwkion/production/5c74153cc4a1bdda72cb9d1ee34b036542c24ea9-6720x4480.jpg/woman-sat-book-reading-room-2024.jpg?w=2560&auto=format/images/v5dwkion/production/5c74153... https://www.bl.uk/images/v5dwkion/production/5c74153cc4a1bdda72cb9d1ee34b036542c24ea9-6720x4480.jpg/woman-sat-book-reading-room-2024.jpg?w=2560&auto=format — 200 — 121.7 KiB — 33 ms — h2https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.js/_next/static/chunks/0s34w9x5095t9.... https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.js — 200 — 7.4 KiB — 96 ms — h2https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.js/_next/static/chunks/0~k6u5_j-9bf2.... https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.js — 200 — 9.2 KiB — 91 ms — h2https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js/_next/static/chunks/0-d5lko7dwx99.... https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js — 200 — 71.4 KiB — 143 ms — h2https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.js/_next/static/chunks/05gp.4a4zhgxr.... https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.js — 200 — 28.9 KiB — 110 ms — h2https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.js/_next/static/chunks/0_9nd31l3ien4.... https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.js — 200 — 11.6 KiB — 127 ms — h2https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.js/_next/static/chunks/turbopack-12qj... https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.js — 200 — 4.6 KiB — 108 ms — h2https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.js/_next/static/chunks/0d~kknpd12z4d.... https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.js — 200 — 7.2 KiB — 103 ms — h2https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js/_next/static/chunks/0fq8sieyn.~r-.... https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js — 200 — 29.3 KiB — 138 ms — h2https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.js/_next/static/chunks/0_dtf~vf7c__z.... https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.js — 200 — 22.3 KiB — 138 ms — h2https://www.bl.uk/_next/static/chunks/1379c4esqhyor.js/_next/static/chunks/1379c4esqhyor.... https://www.bl.uk/_next/static/chunks/1379c4esqhyor.js — 200 — 22.8 KiB — 144 ms — h2https://www.bl.uk/_next/static/chunks/13.otg-600jda.js/_next/static/chunks/13.otg-600jda.... https://www.bl.uk/_next/static/chunks/13.otg-600jda.js — 200 — 69.4 KiB — 88 ms — h2https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js/_next/static/chunks/021g8z6d-ga-i.... https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js — 200 — 214.6 KiB — 147 ms — h2https://www.bl.uk/_next/static/chunks/0i.l9589uvx0j.js/_next/static/chunks/0i.l9589uvx0j.... https://www.bl.uk/_next/static/chunks/0i.l9589uvx0j.js — 200 — 13.2 KiB — 130 ms — h2https://www.bl.uk/_next/static/chunks/0qkb.1tmt8gd6.js/_next/static/chunks/0qkb.1tmt8gd6.... https://www.bl.uk/_next/static/chunks/0qkb.1tmt8gd6.js — 200 — 1.5 KiB — 115 ms — h2https://www.bl.uk/_next/static/chunks/0f71i.89nknfz.js/_next/static/chunks/0f71i.89nknfz.... https://www.bl.uk/_next/static/chunks/0f71i.89nknfz.js — 200 — 1.5 KiB — 124 ms — h2https://www.bl.uk/_next/static/chunks/03jr_-nlu-9by.js/_next/static/chunks/03jr_-nlu-9by.... https://www.bl.uk/_next/static/chunks/03jr_-nlu-9by.js — 200 — 20.3 KiB — 133 ms — h2https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4cdn.sanity.io/files/v5dwkion/produc... https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4 — 206 — 485 B — 28 ms — h2https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4cdn.sanity.io/files/v5dwkion/produc... https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4 — 206 — 23.8 KiB — 110 ms — h2https://www.bl.uk/images/v5dwkion/production/cd6b5dde849375d52477cbf56a9a4eda291197f2-1700x680.webp/fairy-tales-banner.jpg?rect=397,0,907,680&w=600&h=450&auto=format/images/v5dwkion/production/cd6b5dd... https://www.bl.uk/images/v5dwkion/production/cd6b5dde849375d52477cbf56a9a4eda291197f2-1700x680.webp/fairy-tales-banner.jpg?rect=397,0,907,680&w=600&h=450&auto=format — 200 — 56.0 KiB — 35 ms — h2https://www.bl.uk/images/v5dwkion/production/879a0ed7a700c598c5a07f4fc1a426dc55a67951-5760x3840.jpg/treasures-gallery-woman-display.jpg?rect=320,0,5120,3840&w=600&h=450&auto=format/images/v5dwkion/production/879a0ed... https://www.bl.uk/images/v5dwkion/production/879a0ed7a700c598c5a07f4fc1a426dc55a67951-5760x3840.jpg/treasures-gallery-woman-display.jpg?rect=320,0,5120,3840&w=600&h=450&auto=format — 200 — 9.1 KiB — 26 ms — h2https://www.bl.uk/images/v5dwkion/production/7f7ed52ac1c9e315ae80edc463ce5d29fd18fe76-7538x5304.jpg/ezra-collective.jpg?rect=466,0,7072,5304&w=600&h=450&auto=format/images/v5dwkion/production/7f7ed52... https://www.bl.uk/images/v5dwkion/production/7f7ed52ac1c9e315ae80edc463ce5d29fd18fe76-7538x5304.jpg/ezra-collective.jpg?rect=466,0,7072,5304&w=600&h=450&auto=format — 200 — 19.8 KiB — 51 ms — h2
0 ms 500 ms 1.0 s 1.5 s 2.0 s 2.5 s FCP 413 ms LCP 989 mshttps://www.bl.uk// https://www.bl.uk/ — 200 — 52.0 KiB — 1.2 s — h2https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2/_next/static/media/Hind_Variable-s... https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2 — 200 — 23.4 KiB — 106 ms — h2https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.css/_next/static/chunks/12v9u.4p6sjz2.... https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.css — 200 — 1.0 KiB — 67 ms — h2https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.css/_next/static/chunks/0cfu9oxbid~j~.... https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.css — 200 — 13.7 KiB — 105 ms — h2https://www.bl.uk/_next/static/chunks/082s_d0fh06im.css/_next/static/chunks/082s_d0fh06im.... https://www.bl.uk/_next/static/chunks/082s_d0fh06im.css — 200 — 3.5 KiB — 93 ms — h2https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css/_next/static/chunks/0b4yko77qod7i.... https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css — 200 — 937 B — 55 ms — h2https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.css/_next/static/chunks/129ls-rwdg-42.... https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.css — 200 — 1.2 KiB — 99 ms — h2https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GDwww.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD — 200 — 168.5 KiB — 47 ms — h2https://www.bl.uk/images/v5dwkion/production/5c74153cc4a1bdda72cb9d1ee34b036542c24ea9-6720x4480.jpg/woman-sat-book-reading-room-2024.jpg?w=2560&auto=format/images/v5dwkion/production/5c74153... https://www.bl.uk/images/v5dwkion/production/5c74153cc4a1bdda72cb9d1ee34b036542c24ea9-6720x4480.jpg/woman-sat-book-reading-room-2024.jpg?w=2560&auto=format — 200 — 121.7 KiB — 33 ms — h2https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.js/_next/static/chunks/0s34w9x5095t9.... https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.js — 200 — 7.4 KiB — 96 ms — h2https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.js/_next/static/chunks/0~k6u5_j-9bf2.... https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.js — 200 — 9.2 KiB — 91 ms — h2https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js/_next/static/chunks/0-d5lko7dwx99.... https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js — 200 — 71.4 KiB — 143 ms — h2https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.js/_next/static/chunks/05gp.4a4zhgxr.... https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.js — 200 — 28.9 KiB — 110 ms — h2https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.js/_next/static/chunks/0_9nd31l3ien4.... https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.js — 200 — 11.6 KiB — 127 ms — h2https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.js/_next/static/chunks/turbopack-12qj... https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.js — 200 — 4.6 KiB — 108 ms — h2https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.js/_next/static/chunks/0d~kknpd12z4d.... https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.js — 200 — 7.2 KiB — 103 ms — h2https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js/_next/static/chunks/0fq8sieyn.~r-.... https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js — 200 — 29.3 KiB — 138 ms — h2https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.js/_next/static/chunks/0_dtf~vf7c__z.... https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.js — 200 — 22.3 KiB — 138 ms — h2https://www.bl.uk/_next/static/chunks/1379c4esqhyor.js/_next/static/chunks/1379c4esqhyor.... https://www.bl.uk/_next/static/chunks/1379c4esqhyor.js — 200 — 22.8 KiB — 144 ms — h2https://www.bl.uk/_next/static/chunks/13.otg-600jda.js/_next/static/chunks/13.otg-600jda.... https://www.bl.uk/_next/static/chunks/13.otg-600jda.js — 200 — 69.4 KiB — 88 ms — h2https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js/_next/static/chunks/021g8z6d-ga-i.... https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js — 200 — 214.6 KiB — 147 ms — h2https://www.bl.uk/_next/static/chunks/0i.l9589uvx0j.js/_next/static/chunks/0i.l9589uvx0j.... https://www.bl.uk/_next/static/chunks/0i.l9589uvx0j.js — 200 — 13.2 KiB — 130 ms — h2https://www.bl.uk/_next/static/chunks/0qkb.1tmt8gd6.js/_next/static/chunks/0qkb.1tmt8gd6.... https://www.bl.uk/_next/static/chunks/0qkb.1tmt8gd6.js — 200 — 1.5 KiB — 115 ms — h2https://www.bl.uk/_next/static/chunks/0f71i.89nknfz.js/_next/static/chunks/0f71i.89nknfz.... https://www.bl.uk/_next/static/chunks/0f71i.89nknfz.js — 200 — 1.5 KiB — 124 ms — h2https://www.bl.uk/_next/static/chunks/03jr_-nlu-9by.js/_next/static/chunks/03jr_-nlu-9by.... https://www.bl.uk/_next/static/chunks/03jr_-nlu-9by.js — 200 — 20.3 KiB — 133 ms — h2https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4cdn.sanity.io/files/v5dwkion/produc... https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4 — 206 — 485 B — 28 ms — h2https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4cdn.sanity.io/files/v5dwkion/produc... https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4 — 206 — 23.8 KiB — 110 ms — h2https://www.bl.uk/images/v5dwkion/production/cd6b5dde849375d52477cbf56a9a4eda291197f2-1700x680.webp/fairy-tales-banner.jpg?rect=397,0,907,680&w=600&h=450&auto=format/images/v5dwkion/production/cd6b5dd... https://www.bl.uk/images/v5dwkion/production/cd6b5dde849375d52477cbf56a9a4eda291197f2-1700x680.webp/fairy-tales-banner.jpg?rect=397,0,907,680&w=600&h=450&auto=format — 200 — 56.0 KiB — 35 ms — h2https://www.bl.uk/images/v5dwkion/production/879a0ed7a700c598c5a07f4fc1a426dc55a67951-5760x3840.jpg/treasures-gallery-woman-display.jpg?rect=320,0,5120,3840&w=600&h=450&auto=format/images/v5dwkion/production/879a0ed... https://www.bl.uk/images/v5dwkion/production/879a0ed7a700c598c5a07f4fc1a426dc55a67951-5760x3840.jpg/treasures-gallery-woman-display.jpg?rect=320,0,5120,3840&w=600&h=450&auto=format — 200 — 9.1 KiB — 26 ms — h2https://www.bl.uk/images/v5dwkion/production/7f7ed52ac1c9e315ae80edc463ce5d29fd18fe76-7538x5304.jpg/ezra-collective.jpg?rect=466,0,7072,5304&w=600&h=450&auto=format/images/v5dwkion/production/7f7ed52... https://www.bl.uk/images/v5dwkion/production/7f7ed52ac1c9e315ae80edc463ce5d29fd18fe76-7538x5304.jpg/ezra-collective.jpg?rect=466,0,7072,5304&w=600&h=450&auto=format — 200 — 19.8 KiB — 51 ms — h2https://www.bl.uk/images/v5dwkion/production/0473233062d888867f2b82216ec45a0e0bb1dff8-4757x3214.jpg/external-st-pancras-banner-2024.jpg?w=600&auto=format/images/v5dwkion/production/0473233... https://www.bl.uk/images/v5dwkion/production/0473233062d888867f2b82216ec45a0e0bb1dff8-4757x3214.jpg/external-st-pancras-banner-2024.jpg?w=600&auto=format — 200 — 52.0 KiB — 37 ms — h2https://www.bl.uk/images/v5dwkion/production/7d25745bc3c94b585939d164d2994f44964036ac-5594x7808.jpg/kings-library-tower-at-the-british-library-%C2%A9-sam-walton.jpg?w=600&auto=format/images/v5dwkion/production/7d25745... https://www.bl.uk/images/v5dwkion/production/7d25745bc3c94b585939d164d2994f44964036ac-5594x7808.jpg/kings-library-tower-at-the-british-library-%C2%A9-sam-walton.jpg?w=600&auto=format — 200 — 56.7 KiB — 58 ms — h2https://www.bl.uk/images/v5dwkion/production/bb5e89502f50b67a184aa3b0d1ba8c0e16f46a2a-3000x2000.jpg/conservator-with-manuscript.jpg?w=600&auto=format/images/v5dwkion/production/bb5e895... https://www.bl.uk/images/v5dwkion/production/bb5e89502f50b67a184aa3b0d1ba8c0e16f46a2a-3000x2000.jpg/conservator-with-manuscript.jpg?w=600&auto=format — 200 — 19.7 KiB — 44 ms — h2https://www.bl.uk/favicon.ico?favicon.10uocwfl4amop.ico/favicon.ico?favicon.10uocwfl4amop.... https://www.bl.uk/favicon.ico?favicon.10uocwfl4amop.ico — 200 — 1.0 KiB — 26 ms — h2https://www.bl.uk/_next/static/chunks/0nyw5z7nxe_uk.js/_next/static/chunks/0nyw5z7nxe_uk.... https://www.bl.uk/_next/static/chunks/0nyw5z7nxe_uk.js — 200 — 7.0 KiB — 25 ms — h2https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.jscc.cdn.civiccomputing.com/9/cookieC... https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js — 200 — 123.3 KiB — 60 ms — h3https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1h1 — 200 — 143.7 KiB — 45 ms — h2https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1h1 — 200 — 136.5 KiB — 44 ms — h3https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1h1 — 200 — 143.6 KiB — 42 ms — h3https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1h1 — 200 — 171.6 KiB — 51 ms — h3https://pagead2.googlesyndication.com/ccm/collect?rcb=5&frm=0&ae=g&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&ep.ads_data_redaction=0&gtm=45He64h1h1v9171490910za200zd9171490910xea&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115938466~115938469~117266401~118128922&apve=1&apvf=f&apvc=1&tft=1776810462071&tfd=1873pagead2.googlesyndication.com/ccm/c... https://pagead2.googlesyndication.com/ccm/collect?rcb=5&frm=0&ae=g&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&ep.ads_data_redaction=0&gtm=45He64h1h1v9171490910za200zd9171490910xea&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115938466~115938469~117266401~118128922&apve=1&apvf=f&apvc=1&tft=1776810462071&tfd=1873 — 200 — 21 B — 185 ms — h3https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4cdn.sanity.io/files/v5dwkion/produc... https://cdn.sanity.io/files/v5dwkion/production/b741fe29b38293b627aa9419a2346e14c6412c41.mp4/british-library-website-welcome-video-2025-rf26.mp4 — 206 — 2.5 MiB — 142 ms — h2https://apikeys.civiccomputing.com/c/v?d=www.bl.uk&p=CookieControl%20Multi-Site&v=9&k=cca887aa3a3ffc173ccb7f20dcb149ab89a9dda4&format=jsonapikeys.civiccomputing.com/c/v?d=ww... https://apikeys.civiccomputing.com/c/v?d=www.bl.uk&p=CookieControl%20Multi-Site&v=9&k=cca887aa3a3ffc173ccb7f20dcb149ab89a9dda4&format=json — 200 — 609 B — 29 ms — h3https://pagead2.googlesyndication.com/ccm/collect?rcb=4&frm=0&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&gtm=45be64h1h1v9200872335z89171490910za20gzb9171490910zd9171490910xec&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115616985~115938466~115938469~117266400~117384406&apve=1&apvf=f&apvc=0&tids=AW-16474873489&tid=AW-16474873489&tft=1776810462391&tfd=2193pagead2.googlesyndication.com/ccm/c... https://pagead2.googlesyndication.com/ccm/collect?rcb=4&frm=0&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&gtm=45be64h1h1v9200872335z89171490910za20gzb9171490910zd9171490910xec&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115616985~115938466~115938469~117266400~117384406&apve=1&apvf=f&apvc=0&tids=AW-16474873489&tid=AW-16474873489&tft=1776810462391&tfd=2193 — 200 — 21 B — 77 ms — h3https://pagead2.googlesyndication.com/ccm/collect?rcb=18&frm=0&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&gtm=45be64h1h1v9217966939z89171490910za20gzb9171490910zd9171490910xec&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115616985~115938466~115938469~117266401&apve=1&apvf=f&apvc=0&tids=AW-987236887&tid=AW-987236887&tft=1776810462463&tfd=2266pagead2.googlesyndication.com/ccm/c... https://pagead2.googlesyndication.com/ccm/collect?rcb=18&frm=0&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&gtm=45be64h1h1v9217966939z89171490910za20gzb9171490910zd9171490910xec&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115616985~115938466~115938469~117266401&apve=1&apvf=f&apvc=0&tids=AW-987236887&tid=AW-987236887&tft=1776810462463&tfd=2266 — 200 — 21 B — 99 ms — h3https://pagead2.googlesyndication.com/ccm/collect?rcb=14&frm=0&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&gtm=45be64h1h1v9184707491z89171490910za20gzb9171490910zd9171490910xec&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115616985~115938466~115938468~117266401~118463262&apve=1&apvf=f&apvc=0&tids=AW-861090797&tid=AW-861090797&tft=1776810462560&tfd=2363pagead2.googlesyndication.com/ccm/c... https://pagead2.googlesyndication.com/ccm/collect?rcb=14&frm=0&en=page_view&dl=https%3A%2F%2Fwww.bl.uk%2F&scrsrc=www.googletagmanager.com&rnd=1222394306.1776810462&navt=n&npa=1&gtm=45be64h1h1v9184707491z89171490910za20gzb9171490910zd9171490910xec&gcs=G100&gcd=13p3p3p2p5l1&dma_cps=-&dma=1&tag_exp=0~115616985~115938466~115938468~117266401~118463262&apve=1&apvf=f&apvc=0&tids=AW-861090797&tid=AW-861090797&tft=1776810462560&tfd=2363 — 200 — 21 B — 139 ms — h3https://region1.google-analytics.com/g/collect?v=2&tid=G-B8DBRB95KV&gtm=45je64h1h1v867729461z89171490910za20gzb9171490910zd9171490910&_p=1776810461875&gcs=G100&gcd=13p3pPp2p5l1&npa=1&dma_cps=-&dma=1&_eu=EAAAAGA&are=1&cid=511134246.1776810463&frm=0&ir=1&pscdl=denied&rcb=18&sr=800x600&uaa=x86&uab=64&uafvl=Not-A.Brand%3B24.0.0.0%7CChromium%3B146.0.7680.164&uam=&uamb=0&uap=macOS&uapv=10.15.7&uaw=0&ul=en-us&_s=1&tag_exp=0~115938466~115938468~117266401~118463261&sid=1776810462&sct=1&seg=0&dl=https%3A%2F%2Fwww.bl.uk%2F&dt=The%20British%20Library%20-%20the%20national%20Library%20of%20the%20UK&_tu=QA&en=page_view&_fv=1&_nsi=1&_ss=1&up.gtm_version=GTM-TSFX37GD%20%7C%2067&up.page_h1=Welcome%20to%20the%20British%20Library&up.full_referrer=&tfd=2492region1.google-analytics.com/g/coll... https://region1.google-analytics.com/g/collect?v=2&tid=G-B8DBRB95KV&gtm=45je64h1h1v867729461z89171490910za20gzb9171490910zd9171490910&_p=1776810461875&gcs=G100&gcd=13p3pPp2p5l1&npa=1&dma_cps=-&dma=1&_eu=EAAAAGA&are=1&cid=511134246.1776810463&frm=0&ir=1&pscdl=denied&rcb=18&sr=800x600&uaa=x86&uab=64&uafvl=Not-A.Brand%3B24.0.0.0%7CChromium%3B146.0.7680.164&uam=&uamb=0&uap=macOS&uapv=10.15.7&uaw=0&ul=en-us&_s=1&tag_exp=0~115938466~115938468~117266401~118463261&sid=1776810462&sct=1&seg=0&dl=https%3A%2F%2Fwww.bl.uk%2F&dt=The%20British%20Library%20-%20the%20national%20Library%20of%20the%20UK&_tu=QA&en=page_view&_fv=1&_nsi=1&_ss=1&up.gtm_version=GTM-TSFX37GD%20%7C%2067&up.page_h1=Welcome%20to%20the%20British%20Library&up.full_referrer=&tfd=2492 — 204 — 545 B — 35 ms — h2
HTML JavaScript CSS Images Fonts XHR/Fetch Other
Third-Party Script Cost
Per-script blocking time, transfer cost, and cache headers
INFO
75% of JavaScript execution is third-party
First-party Third-party 893ms · 887KB · £9/mo
Google Tag Manager
Tag Manager
Costly
Execution 217ms
Transfer 169 KB
Unused 36%
Monthly Cost £2/mo
Unattributable
Other
Optional
Execution 181ms
Transfer 0 KB
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Optional
Execution 133ms
Transfer 172 KB
Unused 40%
Monthly Cost £1/mo
cc.cdn.civiccomputing.com
Other
Optional
Execution 119ms
Transfer 123 KB
Unused 45%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 93ms
Transfer 144 KB
Unused 46%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 81ms
Transfer 144 KB
Unused 46%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 68ms
Transfer 137 KB
Unused 48%
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 217ms and costs ~£2/month

75% of JavaScript execution time is spent on third-party scripts. Consider auditing which scripts are essential.

Why this matters

When third-party JS execution time exceeds your own, performance gains from frontend work are capped by code you don't own.

Learn more ▾

Every millisecond of third-party JS competes with your own for main-thread time. If their share is bigger than yours, optimization on your code base barely moves the needle. Audit, defer, or remove third-parties before further frontend optimization. Use Lighthouse's third-party audit to identify the worst offenders.

Source: web.dev

Google Tag Manager takes 217ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.

Why this matters

This script has high main-thread execution time — optimize hot paths or defer.

Source: web.dev

How third-party costs are calculated

All checks on this page are automated. Results are estimates - run targeted manual reviews when the score affects a release decision.

Send Feedback