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https://bl.uk

Performance

· 15 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
75
GRADE
C
FIX
5
REVIEW
2
PASS
6
INFO
2

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

Mobile Desktop
96
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.

709 ms

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

1.09 s

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

32 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.

1.54 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.

2.91 s

Page Load Progression

440 ms
881 ms
1.3 s
1.8 s
2.2 s
2.6 s
3.1 s
3.5 s
Checks
15
6 PASS 2 REVIEW 5 FIX
D
Page Weight Budget
Action
2.4 MB transferred, 48 requests
FIX
2.4 MB transferred, 48 requests
Info::
Page weighs 7.7 MB (2.4 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::
48 HTTP requests
Info::
Estimated 0.50 g CO2 per page load
2.4 MB / 2.4 MB budget
2.4 MB transferred
48 requests
0.50 g CO2 per page load
JavaScript 1.4 MiB 58%
Other 586.3 KiB 24%
Images 334.9 KiB 14%
HTML 52.2 KiB 2%
Fonts 23.5 KiB 1%
CSS 20.3 KiB 1%
Other 1.2 KiB 0%
Other 629 B 0%
Other 610 B 0%
0 2.4 MB 4.9 MB
Approaching limit

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
100% third-party, 0 ms blocking
FIX
100% third-party, 0 ms blocking
Warning::
Third-party code accounts for 100% of page weight (2.4 MiB of 2.4 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
F
JS Bundles
Action
23 scripts, 733 KB unused
FIX
23 scripts, 733 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: 733 KB
733 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%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1172 KB69 KB (40%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1144 KB66 KB (46%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1144 KB65 KB (46%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1137 KB65 KB (48%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD169 KB61 KB (37%)✓3rd
https://www.bl.uk/_next/static/chunks/13.otg-600jda.js69 KB60 KB (88%)✓3rd
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%)✓3rd
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js71 KB26 KB (37%)✓3rd

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
48 resources · 2.4 MB · 733 KB savings available
FIX
48 resources · 2.4 MB · 733 KB savings available
Warning::
Page weight 1.4 MB over 1MB target
Got: 2.4 MB Expected: ≤ 1 MB
Info::
~733 KB of savings available
Warning::
47 third-party resources (100% of weight)
Info::
2 resources over 200KB
48 resources · 2.4 MiB · 47 third-party 1.4 MiB over 1MB target
JavaScript 58% · 1.4 MiB
Media 24% · 586.3 KiB
Image 14% · 335.9 KiB
Document 2% · 52.2 KiB
Font 1% · 23.5 KiB
CSS 1% · 20.3 KiB
Other 0% · 1.3 KiB

~733.2 KiB of savings available

Remove unused JavaScript: 733.2 KiB
F
HTTP Caching
Action
No cache headers
FIX
No cache headers
Warning::
No Cache-Control header found
Browsers will use heuristic caching, which can be unpredictable. Set explicit cache headers.

No Cache-Control header

Adding a Cache-Control header can significantly improve repeat-visit performance.

B
JS Execution Cost
518ms total JS execution
REVIEW
518ms total JS execution
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-TSF...: 105ms CPU time
Info::
Unattributable: 82ms CPU time
Info::
https://www.bl.uk/: 78ms CPU time
Info::
https://www.googletagmanager.com/gtag/js?id=G-B8DB...: 68ms CPU time
Info::
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx9...: 63ms CPU time
Warning::
Third-party scripts: 518ms (100% of total)

Main Thread Breakdown

Script Evaluation 57% Script Parsing & Compilation 17% Other 16% Style & Layout 6%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD105ms84ms12ms3rd
Unattributable82ms2ms0ms3rd
https://www.bl.uk/78ms4ms7ms3rd
https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h168ms55ms12ms3rd
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js63ms48ms6ms3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js62ms50ms12ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h161ms49ms11ms3rd
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 709 ms LCP 1.1 shttps://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 — 23.5 KiB — 88 ms — font-display: swap
Hind_Variable-s.p.0238s... woff2 swap
Size 23 KB
Load time 88 ms
Start 303 ms
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
48 requests over 1231ms
INFO
0 ms 500 ms 1.0 s FCP 709 ms LCP 1.1 shttps://bl.uk// https://bl.uk/ — 308 — 137 B — 80 ms — h2https://www.bl.uk/www.bl.uk/ https://www.bl.uk/ — 200 — 52.2 KiB — 401 ms — h2https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2www.bl.uk/_next/static/media/Hind_V... https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2 — 200 — 23.5 KiB — 88 ms — h2https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.csswww.bl.uk/_next/static/chunks/12v9u... https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.css — 200 — 1.0 KiB — 110 ms — h2https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.csswww.bl.uk/_next/static/chunks/0cfu9... https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.css — 200 — 13.7 KiB — 129 ms — h2https://www.bl.uk/_next/static/chunks/082s_d0fh06im.csswww.bl.uk/_next/static/chunks/082s_... https://www.bl.uk/_next/static/chunks/082s_d0fh06im.css — 200 — 3.5 KiB — 104 ms — h2https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.csswww.bl.uk/_next/static/chunks/0b4yk... https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css — 200 — 938 B — 81 ms — h2https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.csswww.bl.uk/_next/static/chunks/129ls... https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.css — 200 — 1.2 KiB — 68 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 — 158 ms — h2https://www.bl.uk/images/v5dwkion/production/5c74153cc4a1bdda72cb9d1ee34b036542c24ea9-6720x4480.jpg/woman-sat-book-reading-room-2024.jpg?w=2560&auto=formatwww.bl.uk/images/v5dwkion/productio... 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 — 55 ms — h2https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.jswww.bl.uk/_next/static/chunks/0s34w... https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.js — 200 — 7.4 KiB — 99 ms — h2https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.jswww.bl.uk/_next/static/chunks/0~k6u... https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.js — 200 — 9.2 KiB — 83 ms — h2https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.jswww.bl.uk/_next/static/chunks/0-d5l... https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js — 200 — 71.4 KiB — 115 ms — h2https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.jswww.bl.uk/_next/static/chunks/05gp.... https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.js — 200 — 28.9 KiB — 104 ms — h2https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.jswww.bl.uk/_next/static/chunks/0_9nd... https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.js — 200 — 11.6 KiB — 102 ms — h2https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.jswww.bl.uk/_next/static/chunks/turbo... https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.js — 200 — 4.6 KiB — 98 ms — h2https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.jswww.bl.uk/_next/static/chunks/0d~kk... https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.js — 200 — 7.2 KiB — 91 ms — h2https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.jswww.bl.uk/_next/static/chunks/0fq8s... https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js — 200 — 29.3 KiB — 119 ms — h2https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.jswww.bl.uk/_next/static/chunks/0_dtf... https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.js — 200 — 22.3 KiB — 126 ms — h2https://www.bl.uk/_next/static/chunks/1379c4esqhyor.jswww.bl.uk/_next/static/chunks/1379c... https://www.bl.uk/_next/static/chunks/1379c4esqhyor.js — 200 — 22.7 KiB — 142 ms — h2https://www.bl.uk/_next/static/chunks/13.otg-600jda.jswww.bl.uk/_next/static/chunks/13.ot... https://www.bl.uk/_next/static/chunks/13.otg-600jda.js — 200 — 69.4 KiB — 122 ms — h2https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.jswww.bl.uk/_next/static/chunks/021g8... https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js — 200 — 214.7 KiB — 128 ms — h2https://www.bl.uk/_next/static/chunks/0i.l9589uvx0j.jswww.bl.uk/_next/static/chunks/0i.l9... https://www.bl.uk/_next/static/chunks/0i.l9589uvx0j.js — 200 — 13.2 KiB — 99 ms — h2https://www.bl.uk/_next/static/chunks/0qkb.1tmt8gd6.jswww.bl.uk/_next/static/chunks/0qkb.... https://www.bl.uk/_next/static/chunks/0qkb.1tmt8gd6.js — 200 — 1.5 KiB — 132 ms — h2https://www.bl.uk/_next/static/chunks/0f71i.89nknfz.jswww.bl.uk/_next/static/chunks/0f71i... https://www.bl.uk/_next/static/chunks/0f71i.89nknfz.js — 200 — 1.5 KiB — 132 ms — h2https://www.bl.uk/_next/static/chunks/03jr_-nlu-9by.jswww.bl.uk/_next/static/chunks/03jr_... https://www.bl.uk/_next/static/chunks/03jr_-nlu-9by.js — 200 — 20.3 KiB — 108 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 — 23.4 KiB — 263 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=formatwww.bl.uk/images/v5dwkion/productio... 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 — 39 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=formatwww.bl.uk/images/v5dwkion/productio... 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 — 38 ms — h2https://www.bl.uk/images/v5dwkion/production/7f7ed52ac1c9e315ae80edc463ce5d29fd18fe76-7538x5304.jpg/ezra-collective.jpg?rect=466,0,7072,5304&w=600&h=450&auto=formatwww.bl.uk/images/v5dwkion/productio... 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 — 39 ms — h2
0 ms 500 ms 1.0 s FCP 709 ms LCP 1.1 shttps://bl.uk// https://bl.uk/ — 308 — 137 B — 80 ms — h2https://www.bl.uk/www.bl.uk/ https://www.bl.uk/ — 200 — 52.2 KiB — 401 ms — h2https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2www.bl.uk/_next/static/media/Hind_V... https://www.bl.uk/_next/static/media/Hind_Variable-s.p.0238s0aaxdc3z.woff2 — 200 — 23.5 KiB — 88 ms — h2https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.csswww.bl.uk/_next/static/chunks/12v9u... https://www.bl.uk/_next/static/chunks/12v9u.4p6sjz2.css — 200 — 1.0 KiB — 110 ms — h2https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.csswww.bl.uk/_next/static/chunks/0cfu9... https://www.bl.uk/_next/static/chunks/0cfu9oxbid~j~.css — 200 — 13.7 KiB — 129 ms — h2https://www.bl.uk/_next/static/chunks/082s_d0fh06im.csswww.bl.uk/_next/static/chunks/082s_... https://www.bl.uk/_next/static/chunks/082s_d0fh06im.css — 200 — 3.5 KiB — 104 ms — h2https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.csswww.bl.uk/_next/static/chunks/0b4yk... https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css — 200 — 938 B — 81 ms — h2https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.csswww.bl.uk/_next/static/chunks/129ls... https://www.bl.uk/_next/static/chunks/129ls-rwdg-42.css — 200 — 1.2 KiB — 68 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 — 158 ms — h2https://www.bl.uk/images/v5dwkion/production/5c74153cc4a1bdda72cb9d1ee34b036542c24ea9-6720x4480.jpg/woman-sat-book-reading-room-2024.jpg?w=2560&auto=formatwww.bl.uk/images/v5dwkion/productio... 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 — 55 ms — h2https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.jswww.bl.uk/_next/static/chunks/0s34w... https://www.bl.uk/_next/static/chunks/0s34w9x5095t9.js — 200 — 7.4 KiB — 99 ms — h2https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.jswww.bl.uk/_next/static/chunks/0~k6u... https://www.bl.uk/_next/static/chunks/0~k6u5_j-9bf2.js — 200 — 9.2 KiB — 83 ms — h2https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.jswww.bl.uk/_next/static/chunks/0-d5l... https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js — 200 — 71.4 KiB — 115 ms — h2https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.jswww.bl.uk/_next/static/chunks/05gp.... https://www.bl.uk/_next/static/chunks/05gp.4a4zhgxr.js — 200 — 28.9 KiB — 104 ms — h2https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.jswww.bl.uk/_next/static/chunks/0_9nd... https://www.bl.uk/_next/static/chunks/0_9nd31l3ien4.js — 200 — 11.6 KiB — 102 ms — h2https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.jswww.bl.uk/_next/static/chunks/turbo... https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.js — 200 — 4.6 KiB — 98 ms — h2https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.jswww.bl.uk/_next/static/chunks/0d~kk... https://www.bl.uk/_next/static/chunks/0d~kknpd12z4d.js — 200 — 7.2 KiB — 91 ms — h2https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.jswww.bl.uk/_next/static/chunks/0fq8s... https://www.bl.uk/_next/static/chunks/0fq8sieyn.~r-.js — 200 — 29.3 KiB — 119 ms — h2https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.jswww.bl.uk/_next/static/chunks/0_dtf... https://www.bl.uk/_next/static/chunks/0_dtf~vf7c__z.js — 200 — 22.3 KiB — 126 ms — h2https://www.bl.uk/_next/static/chunks/1379c4esqhyor.jswww.bl.uk/_next/static/chunks/1379c... https://www.bl.uk/_next/static/chunks/1379c4esqhyor.js — 200 — 22.7 KiB — 142 ms — h2https://www.bl.uk/_next/static/chunks/13.otg-600jda.jswww.bl.uk/_next/static/chunks/13.ot... https://www.bl.uk/_next/static/chunks/13.otg-600jda.js — 200 — 69.4 KiB — 122 ms — 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HTML JavaScript CSS Images Fonts XHR/Fetch Other
Third-Party Script Cost
Per-script blocking time, transfer cost, and cache headers
INFO
100% of JavaScript execution is third-party
First-party Third-party 518ms · 731KB · £5/mo
Google Tag Manager
Tag Manager
Optional
Execution 105ms
Transfer 169 KB
Unused 37%
Monthly Cost £1/mo
Unattributable
Other
Optional
Execution 82ms
Transfer 0 KB
Monthly Cost £1/mo
www.bl.uk
Other
Optional
Execution 78ms
Transfer 52 KB
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 68ms
Transfer 172 KB
Unused 40%
Monthly Cost £1/mo
www.bl.uk
Other
Optional
Execution 63ms
Transfer 71 KB
Unused 37%
Monthly Cost £1/mo
cc.cdn.civiccomputing.com
Other
Optional
Execution 62ms
Transfer 123 KB
Unused 45%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 61ms
Transfer 144 KB
Unused 46%
Monthly Cost £1/mo

100% 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

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.

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