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https://data.gov.uk

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
87
GRADE
B
FIX
2
REVIEW
5
PASS
8
INFO
2

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

Mobile Desktop
51
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.
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.

2.56 s

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

4.66 s

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

202 ms

Cumulative Layout Shift Cumulative Layout Shift — measures visual stability. How much the page layout shifts during loading. Under 0.1 is good.

1.421

Speed Index Speed Index — how quickly content is visually displayed during load. Under 3.4s is good.

2.56 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.

4.66 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 5 REVIEW 2 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 (827.9 KiB of 828.1 KiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
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
10 images, 0 KB saveable
REVIEW
10 images, 0 KB saveable
Info::
All images are well-optimized
10 images 331 KB
0 oversized -0 KB
10 legacy format
0 missing dimensions CLS risk
B
JS Execution Cost
873ms total JS execution
REVIEW
873ms total JS execution
Warning::
https://www.data.gov.uk/find-assets/v2/application...: 294ms CPU time
Info::
https://www.data.gov.uk/: 222ms CPU time
Info::
Unattributable: 217ms CPU time
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-5WR...: 141ms CPU time
Warning::
Third-party scripts: 873ms (100% of total)

Main Thread Breakdown

Script Evaluation 35% Other 28% Style & Layout 19% Script Parsing & Compilation 12%
ScriptTotalScriptingParse/CompileParty
https://www.data.gov.uk/find-assets/v2/application-fefafe876eb73609633cb4fe79695ec11e8f30a8589bcd2c6fdf4511325fcde6.js294ms204ms76ms3rd
https://www.data.gov.uk/222ms8ms2ms3rd
Unattributable217ms6ms0ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5WRWCH8X&141ms103ms35ms3rd
C
JS Bundles
Action
3 scripts, 246 KB unused
REVIEW
3 scripts, 246 KB unused
Warning::
https://www.data.gov.uk/find-assets/v2/application...: 140 KB unused (78%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-5WR...: 67 KB unused (54%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.data.gov.uk/javascript/govuk-frontend-...: 38 KB unused (84%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 246 KB
246 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.data.gov.uk/find-assets/v2/application-fefafe876eb73609633cb4fe79695ec11e8f30a8589bcd2c6fdf4511325fcde6.js179 KB140 KB (78%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5WRWCH8X&126 KB67 KB (54%)✓3rd
https://www.data.gov.uk/javascript/govuk-frontend-6.0.0.min.js46 KB38 KB (84%)✓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

B
Page Weight Inventory
22 resources · 828 KB · 246 KB savings available
REVIEW
22 resources · 828 KB · 246 KB savings available
Info::
Within 1MB page-weight target
Got: 828 KB
Info::
~246 KB of savings available
Warning::
21 third-party resources (100% of weight)
22 resources · 828.1 KiB · 21 third-party Within 1MB page-weight target
JavaScript 42% · 350.6 KiB
Image 40% · 331.3 KiB
Font 12% · 99.6 KiB
CSS 3% · 24.8 KiB
Document 3% · 21.7 KiB
Other 0% · 145 B

~245.7 KiB of savings available

Remove unused JavaScript: 245.7 KiB
C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

A
Page Weight Budget
828 KB transferred, 22 requests
PASS
828 KB transferred, 22 requests
Info::
Page weighs 1.7 MB (828 KB transferred)
Info::
22 HTTP requests
Info::
Estimated 0.17 g CO2 per page load
828 KB / 2.4 MB budget
828 KB transferred
22 requests
0.17 g CO2 per page load
JavaScript 350.6 KiB 42%
Images 330.8 KiB 40%
Fonts 99.6 KiB 12%
CSS 24.8 KiB 3%
HTML 21.7 KiB 3%
Other 664 B 0%
0 2.4 MB 4.9 MB
Under budget
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
4 fonts (100 KB)
PASS
4 fonts (100 KB)
Info::
4 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

4

100 KB total

Render-blocking

0

of 4

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 FCP 2.6 s LCP 4.7 shttps://www.data.gov.uk/find-assets/light-94a07e06a1-v2-eedfb3c2f7945caebd0b15522b59d6c7f01be17fecd6102fd76452ad4042f7b0.woff2light-94a07e06a1-v2-eed... https://www.data.gov.uk/find-assets/light-94a07e06a1-v2-eedfb3c2f7945caebd0b15522b59d6c7f01be17fecd6102fd76452ad4042f7b0.woff2 — 32.7 KiB — 5 ms — font-display: swaphttps://www.data.gov.uk/find-assets/inter-v20-latin-regular-8909904ab6c872eb994093482a88a28eca2cd95912d7b6fecd72103b0dc07edc.woff2inter-v20-latin-regular... https://www.data.gov.uk/find-assets/inter-v20-latin-regular-8909904ab6c872eb994093482a88a28eca2cd95912d7b6fecd72103b0dc07edc.woff2 — 23.3 KiB — 11 ms — font-display: swaphttps://www.data.gov.uk/find-assets/inter-v20-latin-600-f9a06e79cd3a2a20951c0f0e28f66dd0e6d3fda73911d640a2125c8fcb78f21a.woff2inter-v20-latin-600-f9a... https://www.data.gov.uk/find-assets/inter-v20-latin-600-f9a06e79cd3a2a20951c0f0e28f66dd0e6d3fda73911d640a2125c8fcb78f21a.woff2 — 23.9 KiB — 14 ms — font-display: swaphttps://www.data.gov.uk/find-assets/libre-baskerville-v24-latin-regular-3eb80fbca70498126925ce16ff5c20b2fa35e67ae7123ddb46302d2778dfbf0e.woff2libre-baskerville-v24-l... https://www.data.gov.uk/find-assets/libre-baskerville-v24-latin-regular-3eb80fbca70498126925ce16ff5c20b2fa35e67ae7123ddb46302d2778dfbf0e.woff2 — 19.7 KiB — 14 ms — font-display: swap
light-94a07e06a1-v2-eed... woff2 swap
Size 33 KB
Load time 5 ms
Start 135 ms
Risk FOUT — text flashes from fallback to web font
inter-v20-latin-regular... woff2 swap
Size 23 KB
Load time 11 ms
Start 137 ms
Risk FOUT — text flashes from fallback to web font
inter-v20-latin-600-f9a... woff2 swap
Size 24 KB
Load time 14 ms
Start 137 ms
Risk FOUT — text flashes from fallback to web font
libre-baskerville-v24-l... woff2 swap
Size 20 KB
Load time 14 ms
Start 137 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+
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
22 requests over 298ms
INFO
0 ms 1.0 s 2.0 s 3.0 s 4.0 s FCP 2.6 s LCP 4.7 shttps://data.gov.uk// https://data.gov.uk/ — 301 — 145 B — 14 ms — h2https://www.data.gov.uk/www.data.gov.uk/ https://www.data.gov.uk/ — 200 — 21.7 KiB — 30 ms — h2https://www.data.gov.uk/find-assets/v2/govuk-frontend-6.0.0.min-0b4f1b047b982122d1fb986e7cb0114ce068b5c12d3215cf1200a8e45f49ae1f.csswww.data.gov.uk/find-assets/v2/govu... https://www.data.gov.uk/find-assets/v2/govuk-frontend-6.0.0.min-0b4f1b047b982122d1fb986e7cb0114ce068b5c12d3215cf1200a8e45f49ae1f.css — 200 — 14.7 KiB — 17 ms — h2https://www.data.gov.uk/find-assets/v2/application-d977cc135c982e194063c874a5b5cd03556954111607fafd59cd1aa52f786cb5.csswww.data.gov.uk/find-assets/v2/appl... https://www.data.gov.uk/find-assets/v2/application-d977cc135c982e194063c874a5b5cd03556954111607fafd59cd1aa52f786cb5.css — 200 — 10.0 KiB — 5 ms — h2https://www.data.gov.uk/find-assets/v2/application-fefafe876eb73609633cb4fe79695ec11e8f30a8589bcd2c6fdf4511325fcde6.jswww.data.gov.uk/find-assets/v2/appl... https://www.data.gov.uk/find-assets/v2/application-fefafe876eb73609633cb4fe79695ec11e8f30a8589bcd2c6fdf4511325fcde6.js — 200 — 178.8 KiB — 32 ms — h2https://www.data.gov.uk/find-assets/v2/collections/badge-business-and-economy-c2eb9beb047b1f97db66307343bdd77c4c54ec9f9e7bc830d491ab2ade99f0bb.pngwww.data.gov.uk/find-assets/v2/coll... https://www.data.gov.uk/find-assets/v2/collections/badge-business-and-economy-c2eb9beb047b1f97db66307343bdd77c4c54ec9f9e7bc830d491ab2ade99f0bb.png — 200 — 33.6 KiB — 13 ms — h2https://www.data.gov.uk/find-assets/v2/collections/badge-environment-0c106b83ab005b42b32c2fa46bca1be923c5c35a3b2cca9b4398790b4bb41cd6.pngwww.data.gov.uk/find-assets/v2/coll... https://www.data.gov.uk/find-assets/v2/collections/badge-environment-0c106b83ab005b42b32c2fa46bca1be923c5c35a3b2cca9b4398790b4bb41cd6.png — 200 — 44.7 KiB — 19 ms — h2https://www.data.gov.uk/javascript/govuk-frontend-6.0.0.min.jswww.data.gov.uk/javascript/govuk-fr... https://www.data.gov.uk/javascript/govuk-frontend-6.0.0.min.js — 200 — 45.9 KiB — 20 ms — h2https://www.data.gov.uk/find-assets/v2/collections/badge-government-441d78215c73847114f66cb82406c50b4e35df9af6e31f3abc1134bdc097f08e.pngwww.data.gov.uk/find-assets/v2/coll... https://www.data.gov.uk/find-assets/v2/collections/badge-government-441d78215c73847114f66cb82406c50b4e35df9af6e31f3abc1134bdc097f08e.png — 200 — 51.3 KiB — 12 ms — h2https://www.data.gov.uk/find-assets/v2/collections/badge-land-and-property-e6d19aa54a44fa2e0852610fd4a6459888f1dd2863a327c7e0d936085d4d33e7.pngwww.data.gov.uk/find-assets/v2/coll... https://www.data.gov.uk/find-assets/v2/collections/badge-land-and-property-e6d19aa54a44fa2e0852610fd4a6459888f1dd2863a327c7e0d936085d4d33e7.png — 200 — 49.9 KiB — 16 ms — h2https://www.data.gov.uk/find-assets/v2/collections/badge-people-ea1d57564368da33d1ecdbd2f5fcafe76fe4f84bcceb3a6ee80bd453ef70206a.pngwww.data.gov.uk/find-assets/v2/coll... https://www.data.gov.uk/find-assets/v2/collections/badge-people-ea1d57564368da33d1ecdbd2f5fcafe76fe4f84bcceb3a6ee80bd453ef70206a.png — 200 — 51.0 KiB — 19 ms — h2https://www.data.gov.uk/find-assets/v2/logo-88716c60b936e0bded570b436eff6274cf6b65d37abf21ac7ebbf1563ef3a257.svgwww.data.gov.uk/find-assets/v2/logo... https://www.data.gov.uk/find-assets/v2/logo-88716c60b936e0bded570b436eff6274cf6b65d37abf21ac7ebbf1563ef3a257.svg — 200 — 3.4 KiB — 19 ms — h2https://www.data.gov.uk/find-assets/v2/collections/badge-transport-0c2fc879379f035cff33466dba87aa4f02a74ea2e014ea54ac9b5e053fbf2fa3.pngwww.data.gov.uk/find-assets/v2/coll... https://www.data.gov.uk/find-assets/v2/collections/badge-transport-0c2fc879379f035cff33466dba87aa4f02a74ea2e014ea54ac9b5e053fbf2fa3.png — 200 — 48.3 KiB — 27 ms — h2https://www.data.gov.uk/find-assets/v2/icons/arrow-white-right-964a518feb97e05729517ad8dbe5cf077a8e108788ee20697057f9353cb80013.svgwww.data.gov.uk/find-assets/v2/icon... https://www.data.gov.uk/find-assets/v2/icons/arrow-white-right-964a518feb97e05729517ad8dbe5cf077a8e108788ee20697057f9353cb80013.svg — 200 — 309 B — 17 ms — h2https://www.googletagmanager.com/gtm.js?id=GTM-5WRWCH8X&www.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-5WRWCH8X& — 200 — 125.8 KiB — 148 ms — h2https://www.data.gov.uk/find-assets/v2/home-hero-bg-1fb05b73102478ffa32d56dcd3f8548c5ba3ba1a8d15d5dba149634986b03a98.jpgwww.data.gov.uk/find-assets/v2/home... https://www.data.gov.uk/find-assets/v2/home-hero-bg-1fb05b73102478ffa32d56dcd3f8548c5ba3ba1a8d15d5dba149634986b03a98.jpg — 200 — 33.6 KiB — 16 ms — h2https://www.data.gov.uk/find-assets/v2/govuk-crest-7d2c3ed8618f9e43cfe993c3e1a2986000f28f43489b537b75900fefd10437a2.svgwww.data.gov.uk/find-assets/v2/govu... https://www.data.gov.uk/find-assets/v2/govuk-crest-7d2c3ed8618f9e43cfe993c3e1a2986000f28f43489b537b75900fefd10437a2.svg — 200 — 14.6 KiB — 16 ms — h2https://www.data.gov.uk/find-assets/light-94a07e06a1-v2-eedfb3c2f7945caebd0b15522b59d6c7f01be17fecd6102fd76452ad4042f7b0.woff2www.data.gov.uk/find-assets/light-9... https://www.data.gov.uk/find-assets/light-94a07e06a1-v2-eedfb3c2f7945caebd0b15522b59d6c7f01be17fecd6102fd76452ad4042f7b0.woff2 — 200 — 32.7 KiB — 5 ms — h2https://www.data.gov.uk/find-assets/inter-v20-latin-regular-8909904ab6c872eb994093482a88a28eca2cd95912d7b6fecd72103b0dc07edc.woff2www.data.gov.uk/find-assets/inter-v... https://www.data.gov.uk/find-assets/inter-v20-latin-regular-8909904ab6c872eb994093482a88a28eca2cd95912d7b6fecd72103b0dc07edc.woff2 — 200 — 23.3 KiB — 11 ms — h2https://www.data.gov.uk/find-assets/inter-v20-latin-600-f9a06e79cd3a2a20951c0f0e28f66dd0e6d3fda73911d640a2125c8fcb78f21a.woff2www.data.gov.uk/find-assets/inter-v... https://www.data.gov.uk/find-assets/inter-v20-latin-600-f9a06e79cd3a2a20951c0f0e28f66dd0e6d3fda73911d640a2125c8fcb78f21a.woff2 — 200 — 23.9 KiB — 14 ms — h2https://www.data.gov.uk/find-assets/libre-baskerville-v24-latin-regular-3eb80fbca70498126925ce16ff5c20b2fa35e67ae7123ddb46302d2778dfbf0e.woff2www.data.gov.uk/find-assets/libre-b... https://www.data.gov.uk/find-assets/libre-baskerville-v24-latin-regular-3eb80fbca70498126925ce16ff5c20b2fa35e67ae7123ddb46302d2778dfbf0e.woff2 — 200 — 19.7 KiB — 14 ms — h2https://www.data.gov.uk/find-assets/v2/favicon-7b575a40ec1338cf0c982520559cba4ec2baeb907098de9b1c7899b8842a1351.pngwww.data.gov.uk/find-assets/v2/favi... https://www.data.gov.uk/find-assets/v2/favicon-7b575a40ec1338cf0c982520559cba4ec2baeb907098de9b1c7899b8842a1351.png — 200 — 519 B — 5 ms — h2
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 873ms · 326KB · £9/mo
www.data.gov.uk
Other
Costly
Execution 294ms
Transfer 179 KB
Unused 78%
Monthly Cost £3/mo
www.data.gov.uk
Other
Costly
Execution 222ms
Transfer 22 KB
Monthly Cost £2/mo
Unattributable
Other
Costly
Execution 217ms
Transfer 0 KB
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Optional
Execution 141ms
Transfer 126 KB
Unused 54%
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • www.data.gov.uk adds 294ms and costs ~£3/month
  • www.data.gov.uk adds 222ms and costs ~£2/month
  • Unattributable adds 217ms 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.data.gov.uk takes 294ms 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

78% of www.data.gov.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

www.data.gov.uk takes 222ms 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 217ms 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

54% 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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