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https://bl.uk

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
75
GRADE
C
FIX
6
REVIEW
1
PASS
8
INFO
2

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

Mobile Desktop
48
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.

2.11 s

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

6.02 s

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

1.07 s

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.

6.08 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.

13.13 s

Page Load Progression

614 ms
1.2 s
1.8 s
2.5 s
3.1 s
3.7 s
4.3 s
4.9 s
Checks
17
8 PASS 1 REVIEW 6 FIX
D
Page Weight Budget
Action
2.4 MB transferred, 45 requests
FIX
2.4 MB transferred, 45 requests
Info::
Page weighs 7.6 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::
45 HTTP requests
Info::
Estimated 0.49 g CO2 per page load
2.4 MB / 2.4 MB budget
2.4 MB transferred
45 requests
0.49 g CO2 per page load
JavaScript 1.4 MiB 59%
Other 610.4 KiB 25%
Images 274.6 KiB 11%
HTML 52.1 KiB 2%
Fonts 23.4 KiB 1%
CSS 20.3 KiB 1%
Other 1.2 KiB 0%
Other 629 B 0%
Other 607 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
D
JS Execution Cost
Action
2758ms total JS execution
FIX
2758ms total JS execution
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-TSF...: 454ms CPU time
Warning::
Unattributable: 443ms CPU time
Warning::
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx9...: 390ms CPU time
Warning::
https://www.bl.uk/: 321ms CPU time
Warning::
https://www.googletagmanager.com/gtag/js?id=G-B8DB...: 280ms CPU time
Warning::
Third-party scripts: 2758ms (100% of total)

Main Thread Breakdown

Script Evaluation 56% Other 17% Script Parsing & Compilation 16% Style & Layout 7%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD454ms352ms66ms3rd
Unattributable443ms9ms0ms3rd
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js390ms338ms21ms3rd
https://www.bl.uk/321ms20ms32ms3rd
https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1h1280ms213ms56ms3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js225ms183ms41ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1h1208ms173ms32ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1h1185ms148ms36ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1h1169ms129ms39ms3rd
https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js82ms22ms60ms3rd
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-164...: 66 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.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-861...: 65 KB unused (45%)
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%)✓3rd
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-16474873489&cx=c&gtm=4e64h1h1144 KB66 KB (46%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1h1137 KB65 KB (48%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1h1144 KB65 KB (45%)✓3rd
https://www.bl.uk/_next/static/chunks/13.otg-600jda.js69 KB62 KB (89%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD169 KB61 KB (37%)✓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 (36%)✓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
45 resources · 2.4 MB · 734 KB savings available
FIX
45 resources · 2.4 MB · 734 KB savings available
Warning::
Page weight 1.4 MB over 1MB target
Got: 2.4 MB Expected: ≤ 1 MB
Info::
~734 KB of savings available
Warning::
44 third-party resources (100% of weight)
Info::
2 resources over 200KB
45 resources · 2.4 MiB · 44 third-party 1.4 MiB over 1MB target
JavaScript 59% · 1.4 MiB
Media 25% · 610.4 KiB
Image 11% · 275.6 KiB
Document 2% · 52.1 KiB
Font 1% · 23.4 KiB
CSS 1% · 20.3 KiB
Other 0% · 1.3 KiB

~733.6 KiB of savings available

Remove unused JavaScript: 733.6 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.

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
4 images, 0 KB saveable
PASS
4 images, 0 KB saveable
Info::
All images are well-optimized
4 images 275 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 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s FCP 2.1 s LCP 6.0 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 — 23.4 KiB — 110 ms — font-display: swap
Hind_Variable-s.p.0238s... woff2 swap
Size 23 KB
Load time 110 ms
Start 312 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
6 hints, 0 missing preconnects
PASS
6 hints, 0 missing preconnects
Info::
Page uses 6 resource hint(s)

Current Resource Hints

preconnect
1
preload
5
dns-prefetch
—
prefetch
—

6 resource hints configured

A+
Render-Blocking Resources
No render-blocking resources detected
PASS
No render-blocking resources detected
Info::
No render-blocking resources detected in <head>
A+
Third-Party Resources
No third-party resources detected
PASS
No third-party resources detected
Network Waterfall
45 requests over 2599ms
INFO
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s FCP 2.1 s LCP 6.0 shttps://bl.uk// https://bl.uk/ — 308 — 136 B — 82 ms — h2https://www.bl.uk/www.bl.uk/ https://www.bl.uk/ — 200 — 52.1 KiB — 253 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 — 23.4 KiB — 110 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 — 95 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 — 110 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 — 106 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 — 937 B — 84 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 — 97 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 — 1.7 s — 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 — 77 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 — 87 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 — 106 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 — 127 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 — 127 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 — 100 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 — 89 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 — 7.1 KiB — 89 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 — 127 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 — 114 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 — 122 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 — 127 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 — 214.6 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 — 116 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 — 117 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 — 99 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 — 105 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 — 26.0 KiB — 277 ms — h2https://www.bl.uk/images/v5dwkion/production/cd6b5dde849375d52477cbf56a9a4eda291197f2-1700x680.webp/fairy-tales-banner.jpg?rect=397,0,907,680&w=1024&h=768&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=1024&h=768&auto=format — 200 — 99.2 KiB — 32 ms — h2https://www.bl.uk/images/v5dwkion/production/879a0ed7a700c598c5a07f4fc1a426dc55a67951-5760x3840.jpg/treasures-gallery-woman-display.jpg?rect=320,0,5120,3840&w=1024&h=768&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=1024&h=768&auto=format — 200 — 15.8 KiB — 27 ms — h2https://www.bl.uk/images/v5dwkion/production/7f7ed52ac1c9e315ae80edc463ce5d29fd18fe76-7538x5304.jpg/ezra-collective.jpg?rect=466,0,7072,5304&w=1024&h=768&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=1024&h=768&auto=format — 200 — 37.9 KiB — 29 ms — h2
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s FCP 2.1 s LCP 6.0 shttps://bl.uk// https://bl.uk/ — 308 — 136 B — 82 ms — h2https://www.bl.uk/www.bl.uk/ https://www.bl.uk/ — 200 — 52.1 KiB — 253 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 — 23.4 KiB — 110 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 — 95 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 — 110 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 — 106 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 — 937 B — 84 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 — 97 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 — 1.7 s — 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 — 77 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 — 87 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 — 106 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 — 127 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 — 127 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 — 100 ms — h2https://www.bl.uk/_next/static/chunks/turbopack-12qjpr1z9mn03.jswww.bl.uk/_next/static/chunks/turbo... 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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 2758ms · 1226KB · £29/mo
Google Tag Manager
Tag Manager
Costly
Execution 454ms
Transfer 169 KB
Unused 37%
Monthly Cost £5/mo
Unattributable
Other
Costly
Execution 443ms
Transfer 0 KB
Monthly Cost £5/mo
www.bl.uk
Other
Costly
Execution 390ms
Transfer 71 KB
Unused 36%
Monthly Cost £4/mo
www.bl.uk
Other
Costly
Execution 321ms
Transfer 52 KB
Monthly Cost £3/mo
Google Tag Manager
Tag Manager
Costly
Execution 280ms
Transfer 172 KB
Unused 40%
Monthly Cost £3/mo
cc.cdn.civiccomputing.com
Other
Costly
Execution 225ms
Transfer 123 KB
Unused 45%
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Costly
Execution 208ms
Transfer 144 KB
Unused 46%
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Optional
Execution 185ms
Transfer 144 KB
Unused 45%
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Optional
Execution 169ms
Transfer 137 KB
Unused 48%
Monthly Cost £2/mo
www.bl.uk
Other
Optional
Execution 82ms
Transfer 215 KB
Unused 100%
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 454ms and costs ~£5/month
  • Unattributable adds 443ms and costs ~£5/month
  • www.bl.uk adds 390ms and costs ~£4/month
  • www.bl.uk adds 321ms and costs ~£3/month
  • Google Tag Manager adds 280ms and costs ~£3/month
  • cc.cdn.civiccomputing.com adds 225ms and costs ~£2/month
  • Google Tag Manager adds 208ms and costs ~£2/month

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

Google Tag Manager takes 454ms 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

Unattributable takes 443ms 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

www.bl.uk takes 390ms 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

www.bl.uk takes 321ms 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

Google Tag Manager takes 280ms 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

cc.cdn.civiccomputing.com takes 225ms 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

Google Tag Manager takes 208ms 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

100% of www.bl.uk's code is unused. The script may be loading features you don't use.

Why this matters

Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.

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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