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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
82
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.

662 ms

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

2.83 s

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

91 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.

1.42 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.

2.89 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
15
6 PASS 2 REVIEW 5 FIX
F
Page Weight Budget
Action
3.8 MB transferred, 51 requests
FIX
3.8 MB transferred, 51 requests
Warning::
Page weighs 6.2 MB (3.8 MB transferred)
Warning::
Images are 2.6 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 2.6 MB
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::
51 HTTP requests
Info::
Estimated 0.79 g CO2 per page load
3.8 MB / 2.4 MB budget
3.8 MB transferred
51 requests
0.79 g CO2 per page load
Images 2.6 MiB 70%
JavaScript 879.2 KiB 23%
Fonts 179.1 KiB 5%
HTML 64.0 KiB 2%
CSS 22.5 KiB 1%
Other 6.5 KiB 0%
Other 2.6 KiB 0%
Other 0 B 0%
0 2.4 MB 4.9 MB
Over budget

Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.

Why this matters

Image bandwidth is high — modern formats (WebP/AVIF) and resizing typically cut it 50%+.

Source: web.dev

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 (3.8 MiB of 3.8 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
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://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.js44 KB—✓3rd
https://www.nhm.ac.uk/_next/static/chunks/886-7d6523c1202edc02.js10 KB—✓3rd
https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da177152312451611 KB—✓3rd
https://www.nhm.ac.uk/_next/static/chunks/webpack-b7749b26d5a76fc9.js2 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
51 resources · 3.8 MB · 487 KB savings available
FIX
51 resources · 3.8 MB · 487 KB savings available
Warning::
Page weight 2.8 MB over 1MB target
Got: 3.8 MB Expected: ≤ 1 MB
Info::
~487 KB of savings available
Warning::
50 third-party resources (100% of weight)
Info::
6 resources over 200KB
51 resources · 3.8 MiB · 50 third-party 2.8 MiB over 1MB target
Image 70% · 2.6 MiB
JavaScript 23% · 880.0 KiB
Font 5% · 179.1 KiB
Document 2% · 64.0 KiB
CSS 1% · 22.5 KiB
Other 0% · 3.3 KiB

~487.5 KiB of savings available

Remove unused JavaScript: 487.5 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
Image Optimization
13 images, 0 KB saveable
REVIEW
13 images, 0 KB saveable
Info::
All images are well-optimized
13 images 2.6 MB
0 oversized -0 KB
9 legacy format
0 missing dimensions CLS risk
B
JS Execution Cost
915ms total JS execution
REVIEW
915ms total JS execution
Warning::
https://www.nhm.ac.uk/_next/static/chunks/framewor...: 257ms CPU time
Info::
https://www.nhm.ac.uk/: 227ms CPU time
Info::
https://www.nhm.ac.uk/_next/static/chunks/pages/_a...: 125ms CPU time
Info::
Unattributable: 83ms CPU time
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-5TD...: 83ms CPU time
Warning::
Third-party scripts: 915ms (100% of total)

Main Thread Breakdown

Script Evaluation 50% Style & Layout 19% Other 14% Script Parsing & Compilation 11%
ScriptTotalScriptingParse/CompileParty
https://www.nhm.ac.uk/_next/static/chunks/framework-64ad27b21261a9ce.js257ms181ms5ms3rd
https://www.nhm.ac.uk/227ms14ms1ms3rd
https://www.nhm.ac.uk/_next/static/chunks/pages/_app-53d16d5b935d3f3a.js125ms59ms66ms3rd
Unattributable83ms3ms0ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5TDGNT83ms62ms19ms3rd
https://www.nhm.ac.uk/cdn-cgi/challenge-platform/scripts/jsd/main.js77ms74ms1ms3rd
https://assets.adobedtm.com/c2b29812570b/d7d2d2aae38a/launch-e413b49718e8.min.js64ms47ms9ms3rd
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 500 ms 1.0 s 1.5 s 2.0 s 2.5 s FCP 662 ms LCP 2.8 shttps://www.nhm.ac.uk/fonts/NHMWallopText.woff2NHMWallopText.woff2 https://www.nhm.ac.uk/fonts/NHMWallopText.woff2 — 90.6 KiB — 28 ms — font-display: swaphttps://www.nhm.ac.uk/fonts/NHMWallopHeadline.woff2NHMWallopHeadline.woff2 https://www.nhm.ac.uk/fonts/NHMWallopHeadline.woff2 — 88.5 KiB — 35 ms — font-display: swap
NHMWallopText.woff2 woff2 swap
Size 91 KB
Load time 28 ms
Start 168 ms
Risk FOUT — text flashes from fallback to web font
NHMWallopHeadline.woff2 woff2 swap
Size 88 KB
Load time 35 ms
Start 168 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
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
51 requests over 1038ms
INFO
0 ms 500 ms 1.0 s 1.5 s 2.0 s 2.5 s FCP 662 ms LCP 2.8 shttps://nhm.ac.uk// https://nhm.ac.uk/ — 301 — 126 B — 18 ms — h2https://www.nhm.ac.uk/www.nhm.ac.uk/ https://www.nhm.ac.uk/ — 200 — 64.0 KiB — 36 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 — 4 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 — 22 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 — 14 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 — 14 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 — 17 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 — 19 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 — 29 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 — 31 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 — 55 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 — 26 ms — h2https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516static.cloudflareinsights.com/beaco... https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516 — 200 — 10.7 KiB — 39 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 — 179.7 KiB — 282 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 — 23 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 — 25 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 — 557 B — 18 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 — 142 B — 20 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/resized-fern-for-homepage-desktop.jpg.thumb.1920.1920.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.1920.1920.png — 200 — 729.0 KiB — 71 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/visit/Exhibitions/wpy61/wpy61-jelly-smack-summer-tile.jpg.thumb.1920.1920.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.1920.1920.png — 200 — 339.8 KiB — 48 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=1920&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=1920&q=75 — 200 — 49.7 KiB — 59 ms — h2https://www.nhm.ac.uk/fonts/NHMWallopText.woff2www.nhm.ac.uk/fonts/NHMWallopText.w... https://www.nhm.ac.uk/fonts/NHMWallopText.woff2 — 200 — 90.6 KiB — 28 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 — 35 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=1920&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=1920&q=75 — 200 — 242.2 KiB — 32 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=1920&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=1920&q=75 — 200 — 96.6 KiB — 22 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=1920&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=1920&q=75 — 200 — 29.1 KiB — 37 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/t-rex-for-homepage.jpg.thumb.1920.1920.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.1920.1920.png — 200 — 140.7 KiB — 42 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/sequoia-for-homepage.jpg.thumb.1920.1920.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.1920.1920.png — 200 — 175.7 KiB — 50 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/fern-for-homepage.jpg.thumb.1920.1920.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.1920.1920.png — 200 — 182.5 KiB — 55 ms — h2https://www.nhm.ac.uk/content/dam/nhm-www/home/2025-new-home-page-test/blue-whale-for-homepage.jpg.thumb.1920.1920.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.1920.1920.png — 200 — 141.3 KiB — 37 ms — h2
0 ms 500 ms 1.0 s 1.5 s 2.0 s 2.5 s FCP 662 ms LCP 2.8 shttps://nhm.ac.uk// https://nhm.ac.uk/ — 301 — 126 B — 18 ms — h2https://www.nhm.ac.uk/www.nhm.ac.uk/ https://www.nhm.ac.uk/ — 200 — 64.0 KiB — 36 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 — 4 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 — 22 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 — 14 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 — 14 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 — 17 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 — 19 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 — 29 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 — 31 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 — 55 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 — 26 ms — h2https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516static.cloudflareinsights.com/beaco... https://static.cloudflareinsights.com/beacon.min.js/v8c78df7c7c0f484497ecbca7046644da1771523124516 — 200 — 10.7 KiB — 39 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 — 179.7 KiB — 282 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 — 23 ms — h2https://www.nhm.ac.uk/_next/static/chunks/pages/index-b0d422fcae8e50d7.jswww.nhm.ac.uk/_next/static/chunks/p... 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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 915ms · 846KB · £9/mo
www.nhm.ac.uk
Other
Costly
Execution 257ms
Transfer 44 KB
Monthly Cost £3/mo
www.nhm.ac.uk
Other
Costly
Execution 227ms
Transfer 64 KB
Monthly Cost £2/mo
www.nhm.ac.uk
Other
Optional
Execution 125ms
Transfer 486 KB
Unused 75%
Monthly Cost £1/mo
Unattributable
Other
Optional
Execution 83ms
Transfer 0 KB
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 83ms
Transfer 180 KB
Unused 51%
Monthly Cost £1/mo
www.nhm.ac.uk
Other
Optional
Execution 77ms
Transfer 0 KB
Monthly Cost £1/mo
assets.adobedtm.com
Other
Optional
Execution 64ms
Transfer 72 KB
Unused 47%
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • www.nhm.ac.uk adds 257ms and costs ~£3/month
  • www.nhm.ac.uk adds 227ms 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 257ms 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 227ms 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

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

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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