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https://nhm.ac.uk

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
75
GRADE
C
FIX
5
REVIEW
2
PASS
8
INFO
2

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

Mobile Desktop
49
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.
81
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
100
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.

2.06 s

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

8.65 s

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

1.10 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.

3.31 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.

10.60 s

Page Load Progression

375 ms
750 ms
1.1 s
1.5 s
1.9 s
2.2 s
2.6 s
3.0 s
Checks
17
8 PASS 2 REVIEW 5 FIX
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 (1.7 MiB of 1.7 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
F
JS Execution Cost
Action
3659ms total JS execution
FIX
3659ms total JS execution
Warning::
https://www.nhm.ac.uk/: 999ms CPU time
Warning::
https://www.nhm.ac.uk/_next/static/chunks/framewor...: 950ms CPU time
Warning::
https://www.nhm.ac.uk/_next/static/chunks/pages/_a...: 450ms CPU time
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-5TD...: 327ms CPU time
Warning::
Unattributable: 301ms CPU time
Warning::
Third-party scripts: 3659ms (100% of total)

Main Thread Breakdown

Script Evaluation 49% Style & Layout 22% Other 13% Script Parsing & Compilation 10% Parse HTML & CSS 3%
ScriptTotalScriptingParse/CompileParty
https://www.nhm.ac.uk/999ms67ms3ms3rd
https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.js950ms683ms18ms3rd
https://www.nhm.ac.uk/_next/static/chunks/pages/_app-53d16d5b935d3f3a.js450ms230ms217ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5TDGNT327ms220ms72ms3rd
Unattributable301ms9ms0ms3rd
https://www.nhm.ac.uk/cdn-cgi/challenge-platform/scripts/jsd/main.js279ms267ms4ms3rd
https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.js216ms171ms37ms3rd
https://www.nhm.ac.uk/_next/static/chunks/main-4b423ec82b590a3f.js78ms61ms15ms3rd
https://www.nhm.ac.uk/scripts/early-analytics.js59ms48ms7ms3rd
D
JS Bundles
Action
20 scripts, 487 KB unused
FIX
20 scripts, 487 KB unused
Warning::
https://www.nhm.ac.uk/_next/static/chunks/pages/_a...: 363 KB unused (75%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-5TD...: 91 KB unused (51%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://assets.adobedtm.com/c2b29812570b/d7d2d2aae...: 34 KB unused (47%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 487 KB
487 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.nhm.ac.uk/_next/static/chunks/pages/_app-53d16d5b935d3f3a.js486 KB363 KB (75%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5TDGNT180 KB91 KB (51%)✓3rd
https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.js72 KB34 KB (47%)✓3rd
https://www.nhm.ac.uk/_next/static/chunks/pages/index-b0d422fcae8e50d7.js1 KB—✓3rd
https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_ssgManifest.js0 KB—✓3rd
https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da177152312451611 KB—✓3rd
https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.js44 KB—✓3rd
https://www.nhm.ac.uk/_next/static/chunks/webpack-b7749b26d5a76fc9.js2 KB—✓3rd
https://www.nhm.ac.uk/_next/static/chunks/886-7d6523c1202edc02.js10 KB—✓3rd
https://www.nhm.ac.uk/_next/static/chunks/2-3a63044a420995fc.js6 KB—✓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

F
Page Weight Inventory
Action
50 resources · 1.7 MB · 487 KB savings available
FIX
50 resources · 1.7 MB · 487 KB savings available
Warning::
Page weight 745 KB over 1MB target
Got: 1.7 MB Expected: ≤ 1 MB
Info::
~487 KB of savings available
Warning::
49 third-party resources (100% of weight)
Info::
1 resources over 200KB
50 resources · 1.7 MiB · 49 third-party 744.7 KiB over 1MB target
JavaScript 50% · 880.3 KiB
Image 35% · 619.7 KiB
Font 10% · 179.2 KiB
Document 4% · 64.0 KiB
CSS 1% · 22.5 KiB
Other 0% · 3.2 KiB

~487.4 KiB of savings available

Remove unused JavaScript: 487.4 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
Page Weight Budget
Action
1.7 MB transferred, 50 requests
REVIEW
1.7 MB transferred, 50 requests
Info::
Page weighs 4.2 MB (1.7 MB transferred)
Warning::
JavaScript is 879 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 879 KB
Info::
50 HTTP requests
Info::
Estimated 0.36 g CO2 per page load
1.7 MB / 2.4 MB budget
1.7 MB transferred
50 requests
0.36 g CO2 per page load
JavaScript 879.5 KiB 50%
Images 614.7 KiB 35%
Fonts 179.2 KiB 10%
HTML 64.0 KiB 4%
CSS 22.5 KiB 1%
Other 6.5 KiB 0%
Other 2.4 KiB 0%
Other 0 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

B
Image Optimization
12 images, 0 KB saveable
REVIEW
12 images, 0 KB saveable
Info::
All images are well-optimized
12 images 615 KB
0 oversized -0 KB
8 legacy format
0 missing dimensions CLS risk
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+
Font Loading
2 fonts (179 KB)
PASS
2 fonts (179 KB)
Info::
2 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

2

179 KB total

Render-blocking

0

of 2

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 7.0 s 8.0 s 9.0 s FCP 2.1 s LCP 8.7 shttps://www.nhm.ac.uk/fonts/NHMWallopText.woff2NHMWallopText.woff2 https://www.nhm.ac.uk/fonts/NHMWallopText.woff2 — 90.7 KiB — 19 ms — font-display: swaphttps://www.nhm.ac.uk/fonts/NHMWallopHeadline.woff2NHMWallopHeadline.woff2 https://www.nhm.ac.uk/fonts/NHMWallopHeadline.woff2 — 88.5 KiB — 31 ms — font-display: swap
NHMWallopText.woff2 woff2 swap
Size 91 KB
Load time 19 ms
Start 214 ms
Risk FOUT — text flashes from fallback to web font
NHMWallopHeadline.woff2 woff2 swap
Size 88 KB
Load time 31 ms
Start 214 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
3 hints, 0 missing preconnects
PASS
3 hints, 0 missing preconnects
Info::
Page uses 3 resource hint(s)

Current Resource Hints

preconnect
—
preload
2
dns-prefetch
—
prefetch
1

3 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
A+
Green Hosting
Whether the site is served from green-energy infrastructure
PASS

Green Hosting

This site is hosted on green energy infrastructure

Provider: Cloudflare

Network Waterfall
50 requests over 1021ms
INFO
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s 7.0 s 8.0 s 9.0 s FCP 2.1 s LCP 8.7 shttps://nhm.ac.uk// https://nhm.ac.uk/ — 301 — 126 B — 23 ms — h2https://www.nhm.ac.uk/www.nhm.ac.uk/ https://www.nhm.ac.uk/ — 200 — 64.0 KiB — 42 ms — h2https://www.nhm.ac.uk/cdn-cgi/speculationwww.nhm.ac.uk/cdn-cgi/speculation https://www.nhm.ac.uk/cdn-cgi/speculation — 200 — 232 B — 7 ms — h2https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.jsassets.adobedtm.com/c2b29812570b/d7... https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.js — 200 — 71.6 KiB — 44 ms — h2https://www.nhm.ac.uk/_next/static/css/b1b78ee8badc65ec.csswww.nhm.ac.uk/_next/static/css/b1b7... https://www.nhm.ac.uk/_next/static/css/b1b78ee8badc65ec.css — 200 — 21.2 KiB — 21 ms — h2https://www.nhm.ac.uk/_next/static/css/caab7f82a60735b8.csswww.nhm.ac.uk/_next/static/css/caab... https://www.nhm.ac.uk/_next/static/css/caab7f82a60735b8.css — 200 — 1.3 KiB — 20 ms — h2https://www.nhm.ac.uk/scripts/early-analytics.jswww.nhm.ac.uk/scripts/early-analyti... https://www.nhm.ac.uk/scripts/early-analytics.js — 200 — 16.1 KiB — 22 ms — h2https://www.nhm.ac.uk/_next/static/chunks/webpack-b7749b26d5a76fc9.jswww.nhm.ac.uk/_next/static/chunks/w... https://www.nhm.ac.uk/_next/static/chunks/webpack-b7749b26d5a76fc9.js — 200 — 1.8 KiB — 33 ms — h2https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.jswww.nhm.ac.uk/_next/static/chunks/f... https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.js — 200 — 44.0 KiB — 35 ms — h2https://www.nhm.ac.uk/_next/static/chunks/main-4b423ec82b590a3f.jswww.nhm.ac.uk/_next/static/chunks/m... https://www.nhm.ac.uk/_next/static/chunks/main-4b423ec82b590a3f.js — 200 — 33.4 KiB — 36 ms — h2https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516static.cloudflareinsights.com/beaco... https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516 — 200 — 10.7 KiB — 62 ms — h2https://www.googletagmanager.com/gtm.js?id=GTM-5TDGNTwww.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-5TDGNT — 200 — 180.0 KiB — 295 ms — h2https://www.nhm.ac.uk/_next/static/chunks/pages/_app-53d16d5b935d3f3a.jswww.nhm.ac.uk/_next/static/chunks/p... https://www.nhm.ac.uk/_next/static/chunks/pages/_app-53d16d5b935d3f3a.js — 200 — 486.2 KiB — 58 ms — h2https://www.nhm.ac.uk/_next/static/chunks/886-7d6523c1202edc02.jswww.nhm.ac.uk/_next/static/chunks/8... https://www.nhm.ac.uk/_next/static/chunks/886-7d6523c1202edc02.js — 200 — 10.3 KiB — 27 ms — h2https://www.nhm.ac.uk/_next/static/chunks/2-3a63044a420995fc.jswww.nhm.ac.uk/_next/static/chunks/2... https://www.nhm.ac.uk/_next/static/chunks/2-3a63044a420995fc.js — 200 — 6.2 KiB — 36 ms — h2https://www.nhm.ac.uk/_next/static/chunks/pages/index-b0d422fcae8e50d7.jswww.nhm.ac.uk/_next/static/chunks/p... https://www.nhm.ac.uk/_next/static/chunks/pages/index-b0d422fcae8e50d7.js — 200 — 698 B — 32 ms — h2https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_buildManifest.jswww.nhm.ac.uk/_next/static/jIUk2ai3... https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_buildManifest.js — 200 — 534 B — 33 ms — h2https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_ssgManifest.jswww.nhm.ac.uk/_next/static/jIUk2ai3... https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_ssgManifest.js — 200 — 136 B — 29 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/resized-fern-for-homepage-desktop.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/resized-fern-for-homepage-desktop.jpg.thumb.480.480.png — 200 — 121.0 KiB — 27 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/visit/Exhibitions/wpy61/wpy61-jelly-smack-summer-tile.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/v... https://www.nhm.ac.uk/content/dam/nhm-www/visit/Exhibitions/wpy61/wpy61-jelly-smack-summer-tile.jpg.thumb.480.480.png — 200 — 34.9 KiB — 31 ms — h2https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-www%2Fvisit%2FExhibitions%2Fvisions-of-nature%2FVisions_of_Nature_mobile.jpg&w=750&q=75www.nhm.ac.uk/_next/image?url=https... https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-www%2Fvisit%2FExhibitions%2Fvisions-of-nature%2FVisions_of_Nature_mobile.jpg&w=750&q=75 — 200 — 49.7 KiB — 28 ms — h2https://www.nhm.ac.uk/fonts/NHMWallopText.woff2www.nhm.ac.uk/fonts/NHMWallopText.w... https://www.nhm.ac.uk/fonts/NHMWallopText.woff2 — 200 — 90.7 KiB — 19 ms — h2https://www.nhm.ac.uk/fonts/NHMWallopHeadline.woff2www.nhm.ac.uk/fonts/NHMWallopHeadli... https://www.nhm.ac.uk/fonts/NHMWallopHeadline.woff2 — 200 — 88.5 KiB — 31 ms — h2https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-www%2Fvisit%2FExhibitions%2Four-story-david-attenborough%2FOur-Story-1500-x-700.jpg&w=750&q=75www.nhm.ac.uk/_next/image?url=https... https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-www%2Fvisit%2FExhibitions%2Four-story-david-attenborough%2FOur-Story-1500-x-700.jpg&w=750&q=75 — 200 — 81.1 KiB — 34 ms — h2https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-events%2FVISEV-DSK%2Fdsk-carousel-4.jpg&w=750&q=75www.nhm.ac.uk/_next/image?url=https... https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-events%2FVISEV-DSK%2Fdsk-carousel-4.jpg&w=750&q=75 — 200 — 66.7 KiB — 76 ms — h2https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-events%2FEXH-JUROC%2FEXH-JUROC.jpg&w=750&q=75www.nhm.ac.uk/_next/image?url=https... https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-events%2FEXH-JUROC%2FEXH-JUROC.jpg&w=750&q=75 — 200 — 29.1 KiB — 22 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/t-rex-for-homepage.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/t-rex-for-homepage.jpg.thumb.480.480.png — 200 — 32.7 KiB — 25 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/sequoia-for-homepage.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/sequoia-for-homepage.jpg.thumb.480.480.png — 200 — 40.2 KiB — 19 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/fern-for-homepage.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/fern-for-homepage.jpg.thumb.480.480.png — 200 — 48.3 KiB — 22 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/blue-whale-for-homepage.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/blue-whale-for-homepage.jpg.thumb.480.480.png — 200 — 39.1 KiB — 17 ms — h2
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s 7.0 s 8.0 s 9.0 s FCP 2.1 s LCP 8.7 shttps://nhm.ac.uk// https://nhm.ac.uk/ — 301 — 126 B — 23 ms — h2https://www.nhm.ac.uk/www.nhm.ac.uk/ https://www.nhm.ac.uk/ — 200 — 64.0 KiB — 42 ms — h2https://www.nhm.ac.uk/cdn-cgi/speculationwww.nhm.ac.uk/cdn-cgi/speculation https://www.nhm.ac.uk/cdn-cgi/speculation — 200 — 232 B — 7 ms — h2https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.jsassets.adobedtm.com/c2b29812570b/d7... https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.js — 200 — 71.6 KiB — 44 ms — h2https://www.nhm.ac.uk/_next/static/css/b1b78ee8badc65ec.csswww.nhm.ac.uk/_next/static/css/b1b7... https://www.nhm.ac.uk/_next/static/css/b1b78ee8badc65ec.css — 200 — 21.2 KiB — 21 ms — h2https://www.nhm.ac.uk/_next/static/css/caab7f82a60735b8.csswww.nhm.ac.uk/_next/static/css/caab... https://www.nhm.ac.uk/_next/static/css/caab7f82a60735b8.css — 200 — 1.3 KiB — 20 ms — h2https://www.nhm.ac.uk/scripts/early-analytics.jswww.nhm.ac.uk/scripts/early-analyti... https://www.nhm.ac.uk/scripts/early-analytics.js — 200 — 16.1 KiB — 22 ms — h2https://www.nhm.ac.uk/_next/static/chunks/webpack-b7749b26d5a76fc9.jswww.nhm.ac.uk/_next/static/chunks/w... https://www.nhm.ac.uk/_next/static/chunks/webpack-b7749b26d5a76fc9.js — 200 — 1.8 KiB — 33 ms — h2https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.jswww.nhm.ac.uk/_next/static/chunks/f... https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.js — 200 — 44.0 KiB — 35 ms — h2https://www.nhm.ac.uk/_next/static/chunks/main-4b423ec82b590a3f.jswww.nhm.ac.uk/_next/static/chunks/m... https://www.nhm.ac.uk/_next/static/chunks/main-4b423ec82b590a3f.js — 200 — 33.4 KiB — 36 ms — h2https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516static.cloudflareinsights.com/beaco... 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h2https://www.nhm.ac.uk/_next/static/chunks/pages/index-b0d422fcae8e50d7.jswww.nhm.ac.uk/_next/static/chunks/p... https://www.nhm.ac.uk/_next/static/chunks/pages/index-b0d422fcae8e50d7.js — 200 — 698 B — 32 ms — h2https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_buildManifest.jswww.nhm.ac.uk/_next/static/jIUk2ai3... https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_buildManifest.js — 200 — 534 B — 33 ms — h2https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_ssgManifest.jswww.nhm.ac.uk/_next/static/jIUk2ai3... https://www.nhm.ac.uk/_next/static/jIUk2ai3GE_NNa0A2G68-/_ssgManifest.js — 200 — 136 B — 29 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/resized-fern-for-homepage-desktop.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/resized-fern-for-homepage-desktop.jpg.thumb.480.480.png — 200 — 121.0 KiB — 27 ms — 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h2https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-events%2FEXH-JUROC%2FEXH-JUROC.jpg&w=750&q=75www.nhm.ac.uk/_next/image?url=https... https://www.nhm.ac.uk/_next/image?url=https%3A%2F%2Fwww.nhm.ac.uk%2Fcontent%2Fdam%2Fnhm-events%2FEXH-JUROC%2FEXH-JUROC.jpg&w=750&q=75 — 200 — 29.1 KiB — 22 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/t-rex-for-homepage.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/t-rex-for-homepage.jpg.thumb.480.480.png — 200 — 32.7 KiB — 25 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/sequoia-for-homepage.jpg.thumb.480.480.pngwww.nhm.ac.uk/content/dam/nhm-www/h... https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/sequoia-for-homepage.jpg.thumb.480.480.png — 200 — 40.2 KiB — 19 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 3659ms · 896KB · £38/mo
www.nhm.ac.uk
Other
Costly
Execution 999ms
Transfer 64 KB
Monthly Cost £10/mo
www.nhm.ac.uk
Other
Costly
Execution 950ms
Transfer 44 KB
Monthly Cost £10/mo
www.nhm.ac.uk
Other
Costly
Execution 450ms
Transfer 486 KB
Unused 75%
Monthly Cost £5/mo
Google Tag Manager
Tag Manager
Costly
Execution 327ms
Transfer 180 KB
Unused 51%
Monthly Cost £3/mo
Unattributable
Other
Costly
Execution 301ms
Transfer 0 KB
Monthly Cost £3/mo
www.nhm.ac.uk
Other
Costly
Execution 279ms
Transfer 0 KB
Monthly Cost £3/mo
assets.adobedtm.com
Other
Costly
Execution 216ms
Transfer 72 KB
Unused 47%
Monthly Cost £2/mo
www.nhm.ac.uk
Other
Optional
Execution 78ms
Transfer 33 KB
Monthly Cost £1/mo
www.nhm.ac.uk
Other
Optional
Execution 59ms
Transfer 16 KB
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • www.nhm.ac.uk adds 999ms and costs ~£10/month
  • www.nhm.ac.uk adds 950ms and costs ~£10/month
  • www.nhm.ac.uk adds 450ms and costs ~£5/month
  • Google Tag Manager adds 327ms and costs ~£3/month
  • Unattributable adds 301ms and costs ~£3/month
  • www.nhm.ac.uk adds 279ms and costs ~£3/month
  • assets.adobedtm.com adds 216ms 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

www.nhm.ac.uk takes 999ms 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.nhm.ac.uk takes 950ms 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.nhm.ac.uk takes 450ms 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

75% of www.nhm.ac.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

Google Tag Manager takes 327ms 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

51% of Google Tag Manager'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

Unattributable takes 301ms 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.nhm.ac.uk takes 279ms 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

assets.adobedtm.com takes 216ms 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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