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Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
70
GRADE
C
FIX
6
REVIEW
1
PASS
8
INFO
2

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

Mobile Desktop
34
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.

3.52 s

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

12.97 s

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

2.49 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.

5.93 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.

13.01 s

Page Load Progression

584 ms
1.2 s
1.8 s
2.3 s
2.9 s
3.5 s
4.1 s
4.7 s
Checks
17
8 PASS 1 REVIEW 6 FIX
F
Page Weight Budget
Action
4.2 MB transferred, 44 requests
FIX
4.2 MB transferred, 44 requests
Critical::
Page weighs 7.7 MB (4.2 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::
44 HTTP requests
Info::
Estimated 0.88 g CO2 per page load
4.2 MB / 2.4 MB budget
4.2 MB transferred
44 requests
0.88 g CO2 per page load
Other 2.4 MiB 58%
JavaScript 1.4 MiB 33%
Images 274.6 KiB 6%
HTML 51.9 KiB 1%
Fonts 23.5 KiB 1%
CSS 20.3 KiB 0%
Other 1.0 KiB 0%
Other 649 B 0%
Other 629 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
79% third-party, 0 ms blocking
FIX
79% third-party, 0 ms blocking
Warning::
Third-party code accounts for 79% of page weight (3.3 MiB of 4.2 MiB)
Info::
Third-party blocking time is low (0 ms)
21%
79%
First-party Third-party
F
JS Execution Cost
Action
4671ms total JS execution
FIX
4671ms total JS execution
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-TSF...: 815ms CPU time
Warning::
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx9...: 692ms CPU time
Warning::
Unattributable: 594ms CPU time
Warning::
https://cc.cdn.civiccomputing.com/9/cookieControl-...: 526ms CPU time
Warning::
https://www.googletagmanager.com/gtag/js?id=G-B8DB...: 516ms CPU time
Warning::
Third-party scripts: 3358ms (72% of total)

Main Thread Breakdown

Script Evaluation 59% Script Parsing & Compilation 18% Other 15% Style & Layout 4%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD815ms615ms131ms3rd
https://www.bl.uk/_next/static/chunks/0-d5lko7dwx99.js692ms596ms45ms1st
Unattributable594ms12ms0ms3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js526ms422ms103ms3rd
https://www.googletagmanager.com/gtag/js?id=G-B8DBRB95KV&cx=c&gtm=4e64h1516ms369ms83ms3rd
https://www.bl.uk/412ms25ms44ms1st
https://www.googletagmanager.com/gtag/js?id=AW-16474873489&cx=c&gtm=4e64h1322ms253ms67ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-861090797&cx=c&gtm=4e64h1313ms255ms56ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1272ms205ms65ms3rd
https://www.bl.uk/_next/static/chunks/021g8z6d-ga-i.js148ms39ms109ms1st

Showing top 10 of 11 scripts

F
JS Bundles
Action
23 scripts, 735 KB unused
FIX
23 scripts, 735 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...: 66 KB unused (46%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-987...: 66 KB unused (48%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 735 KB
735 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=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 KB66 KB (46%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-987236887&cx=c&gtm=4e64h1137 KB66 KB (48%)✓3rd
https://www.bl.uk/_next/static/chunks/13.otg-600jda.js69 KB62 KB (89%)✓1st
https://www.googletagmanager.com/gtm.js?id=GTM-TSFX37GD169 KB61 KB (36%)✓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%)✓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
44 resources · 4.2 MB · 735 KB savings available
FIX
44 resources · 4.2 MB · 735 KB savings available
Warning::
Page weight 3.2 MB over 1MB target
Got: 4.2 MB Expected: ≤ 1 MB
Info::
~735 KB of savings available
Warning::
15 third-party resources (79% of weight)
Info::
2 resources over 200KB
44 resources · 4.2 MiB · 15 third-party 3.2 MiB over 1MB target
Media 58% · 2.4 MiB
JavaScript 33% · 1.4 MiB
Image 6% · 275.6 KiB
Document 1% · 51.9 KiB
Font 1% · 23.5 KiB
CSS 0% · 20.3 KiB
Other 0% · 1.2 KiB

~735.0 KiB of savings available

Remove unused JavaScript: 735.0 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
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 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s FCP 3.5 s LCP 13.0 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 — 23.5 KiB — 189 ms — font-display: swap
Hind_Variable-s.p.0238s... woff2 swap
Size 23 KB
Load time 189 ms
Start 181 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
44 requests over 1436ms
INFO
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s FCP 3.5 s LCP 13.0 shttps://www.bl.uk// https://www.bl.uk/ — 200 — 51.9 KiB — 194 ms — 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 — 23.5 KiB — 189 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 — 115 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 — 125 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 — 120 ms — h2https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css/_next/static/chunks/0b4yko77qod7i.... https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css — 200 — 938 B — 49 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 — 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 — 54 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 — 25 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 — 129 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 — 101 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 — 175 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 — 141 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 — 156 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 — 91 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 — 7.1 KiB — 106 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 — 171 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 — 128 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 — 146 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 — 189 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 — 196 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 — 111 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 — 126 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 — 114 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 — 179 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 — 466 B — 36 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 — 153 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 — 33 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 — 39 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 — 123.4 KiB — 58 ms — h3
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s FCP 3.5 s LCP 13.0 shttps://www.bl.uk// https://www.bl.uk/ — 200 — 51.9 KiB — 194 ms — 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 — 23.5 KiB — 189 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 — 115 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 — 125 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 — 120 ms — h2https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css/_next/static/chunks/0b4yko77qod7i.... https://www.bl.uk/_next/static/chunks/0b4yko77qod7i.css — 200 — 938 B — 49 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 — 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 — 54 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 — 25 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 — 129 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 — 101 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 — 175 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 — 141 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 — 156 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 — 91 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
72% of JavaScript execution is third-party
First-party Third-party 3358ms · 887KB · £35/mo
Google Tag Manager
Tag Manager
Costly
Execution 815ms
Transfer 169 KB
Unused 36%
Monthly Cost £8/mo
Unattributable
Other
Costly
Execution 594ms
Transfer 0 KB
Monthly Cost £6/mo
cc.cdn.civiccomputing.com
Other
Costly
Execution 526ms
Transfer 123 KB
Unused 45%
Monthly Cost £5/mo
Google Tag Manager
Tag Manager
Costly
Execution 516ms
Transfer 172 KB
Unused 40%
Monthly Cost £5/mo
Google Tag Manager
Tag Manager
Costly
Execution 322ms
Transfer 144 KB
Unused 46%
Monthly Cost £3/mo
Google Tag Manager
Tag Manager
Costly
Execution 313ms
Transfer 144 KB
Unused 46%
Monthly Cost £3/mo
Google Tag Manager
Tag Manager
Costly
Execution 272ms
Transfer 137 KB
Unused 48%
Monthly Cost £3/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 815ms and costs ~£8/month
  • Unattributable adds 594ms and costs ~£6/month
  • cc.cdn.civiccomputing.com adds 526ms and costs ~£5/month
  • Google Tag Manager adds 516ms and costs ~£5/month
  • Google Tag Manager adds 322ms and costs ~£3/month
  • Google Tag Manager adds 313ms and costs ~£3/month
  • Google Tag Manager adds 272ms and costs ~£3/month

72% 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 815ms 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 594ms 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 526ms 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 516ms 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 322ms 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 313ms 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 272ms 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.

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