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https://ons.gov.uk

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
84
GRADE
B
FIX
2
REVIEW
6
PASS
7
INFO
2

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

Mobile Desktop
69
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.
77
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
91
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.

2.78 s

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

5.51 s

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

266 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.

3.33 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.

5.56 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
7 PASS 6 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 (1.3 MiB of 1.3 MiB)
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
Page Weight Budget
1.3 MB transferred, 26 requests
REVIEW
1.3 MB transferred, 26 requests
Info::
Page weighs 2.6 MB (1.3 MB transferred)
Info::
26 HTTP requests
Info::
Estimated 0.27 g CO2 per page load
1.3 MB / 2.4 MB budget
1.3 MB transferred
26 requests
0.27 g CO2 per page load
Images 893.3 KiB 68%
JavaScript 318.9 KiB 24%
CSS 47.8 KiB 4%
Fonts 35.7 KiB 3%
HTML 13.7 KiB 1%
Other 1.9 KiB 0%
Other 973 B 0%
0 2.4 MB 4.9 MB
Under budget
C
Image Optimization
Action
13 images, 0 KB saveable
REVIEW
13 images, 0 KB saveable
Warning::
https://static.ons.gov.uk/images/fb9be62f-1124-43a... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/835f0472-877e-4ad... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/218433dd-b40c-44c... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://cdn.ons.gov.uk/assets/images/logo-ogl-foot... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://cdn.ons.gov.uk/assets/images/ons-logo/v2/o... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://cdn.ons.gov.uk/assets/images/icon-checkbox... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/0f82a37b-7cb8-484... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/a891289e-3abd-402... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/0d844d98-3f70-4c2... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/49166bf5-1f2a-4e7... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://static.ons.gov.uk/images/9eba40bd-f6c2-41d... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
13 images 893 KB
0 oversized -0 KB
13 legacy format
11 missing dimensions CLS risk

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

Set explicit width and height to prevent CLS.

Why this matters

Image without explicit width/height — browser can't reserve space; CLS jumps when image loads.

Source: web.dev / Core Web Vitals

C
JS Execution Cost
Action
1337ms total JS execution
REVIEW
1337ms total JS execution
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-MBC...: 440ms CPU time
Warning::
https://www.ons.gov.uk/: 262ms CPU time
Info::
https://www.ons.gov.uk/cdn-cgi/challenge-platform/...: 245ms CPU time
Info::
Unattributable: 186ms CPU time
Info::
https://ajax.googleapis.com/ajax/libs/jquery/3.7.1...: 122ms CPU time
Warning::
Third-party scripts: 1337ms (100% of total)

Main Thread Breakdown

Script Evaluation 57% Other 16% Style & Layout 14% Script Parsing & Compilation 7% Parse HTML & CSS 4%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-MBCBVQS440ms381ms45ms3rd
https://www.ons.gov.uk/262ms41ms4ms3rd
https://www.ons.gov.uk/cdn-cgi/challenge-platform/scripts/jsd/main.js245ms223ms2ms3rd
Unattributable186ms6ms0ms3rd
https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js122ms92ms8ms3rd
https://www.ons.gov.uk/js/app.js82ms41ms33ms3rd
B
JS Bundles
5 scripts, 143 KB unused
REVIEW
5 scripts, 143 KB unused
Warning::
https://www.ons.gov.uk/js/app.js: 92 KB unused (83%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-MBC...: 51 KB unused (34%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 143 KB
143 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.ons.gov.uk/js/app.js110 KB92 KB (83%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-MBCBVQS149 KB51 KB (34%)✓3rd
https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js30 KB—✓3rd
https://cdn.ons.gov.uk/sixteens/418c927/js/main.js17 KB—✓3rd
https://www.ons.gov.uk/cdn-cgi/challenge-platform/h/b/scripts/jsd/0b8fb825cb67/main.js?12 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

C
Page Weight Inventory
Action
26 resources · 1.3 MB · 143 KB savings available
REVIEW
26 resources · 1.3 MB · 143 KB savings available
Warning::
Page weight 288 KB over 1MB target
Got: 1.3 MB Expected: ≤ 1 MB
Info::
~143 KB of savings available
Warning::
25 third-party resources (100% of weight)
Info::
1 resources over 200KB
26 resources · 1.3 MiB · 25 third-party 288.3 KiB over 1MB target
Image 68% · 894.7 KiB
JavaScript 24% · 318.9 KiB
CSS 4% · 47.8 KiB
Font 3% · 35.7 KiB
Document 1% · 13.9 KiB
Other 0% · 1.3 KiB

~142.9 KiB of savings available

Remove unused JavaScript: 142.9 KiB
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+
Font Loading
2 fonts (36 KB)
PASS
2 fonts (36 KB)
Info::
2 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

2

36 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 FCP 2.8 s LCP 5.5 shttps://cdn.ons.gov.uk/assets/fonts/open-sans-regular/OpenSans-Regular-webfont.woff2OpenSans-Regular-webfon... https://cdn.ons.gov.uk/assets/fonts/open-sans-regular/OpenSans-Regular-webfont.woff2 — 18.1 KiB — 69 ms — font-display: swaphttps://cdn.ons.gov.uk/assets/fonts/open-sans-bold/OpenSans-Bold-webfont.woff2OpenSans-Bold-webfont.w... https://cdn.ons.gov.uk/assets/fonts/open-sans-bold/OpenSans-Bold-webfont.woff2 — 17.6 KiB — 70 ms — font-display: swap
OpenSans-Regular-webfon... woff2 swap
Size 18 KB
Load time 69 ms
Start 366 ms
Risk FOUT — text flashes from fallback to web font
OpenSans-Bold-webfont.w... woff2 swap
Size 18 KB
Load time 70 ms
Start 367 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
26 requests over 714ms
INFO
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s FCP 2.8 s LCP 5.5 shttps://ons.gov.uk// https://ons.gov.uk/ — 301 — 191 B — 83 ms — http/1.1https://www.ons.gov.uk/www.ons.gov.uk/ https://www.ons.gov.uk/ — 200 — 13.7 KiB — 79 ms — h2https://cdn.ons.gov.uk/sixteens/418c927/css/main.csscdn.ons.gov.uk/sixteens/418c927/css... https://cdn.ons.gov.uk/sixteens/418c927/css/main.css — 200 — 47.8 KiB — 130 ms — h2https://cdn.ons.gov.uk/assets/images/ons-logo/v2/ons-logo.svgcdn.ons.gov.uk/assets/images/ons-lo... https://cdn.ons.gov.uk/assets/images/ons-logo/v2/ons-logo.svg — 200 — 2.5 KiB — 126 ms — h2https://cdn.ons.gov.uk/assets/images/icon-checkbox.svgcdn.ons.gov.uk/assets/images/icon-c... https://cdn.ons.gov.uk/assets/images/icon-checkbox.svg — 200 — 651 B — 129 ms — h2https://static.ons.gov.uk/images/9eba40bd-f6c2-41d8-8dcd-7eb67c184f05/original/Homepage-thumbnail_Hero-video_v2.pngstatic.ons.gov.uk/images/9eba40bd-f... https://static.ons.gov.uk/images/9eba40bd-f6c2-41d8-8dcd-7eb67c184f05/original/Homepage-thumbnail_Hero-video_v2.png — 200 — 139.7 KiB — 173 ms — h2https://static.ons.gov.uk/images/0f82a37b-7cb8-4840-9ddc-31738be805de/original/Data-coherence.pngstatic.ons.gov.uk/images/0f82a37b-7... https://static.ons.gov.uk/images/0f82a37b-7cb8-4840-9ddc-31738be805de/original/Data-coherence.png — 200 — 46.1 KiB — 128 ms — h2https://cdn.ons.gov.uk/assets/images/census-logo/logo-census-2021-white-landscape.svgcdn.ons.gov.uk/assets/images/census... https://cdn.ons.gov.uk/assets/images/census-logo/logo-census-2021-white-landscape.svg — 200 — 3.2 KiB — 33 ms — h2https://static.ons.gov.uk/images/a891289e-3abd-4027-bc98-a674d311db56/original/2912%20Census%202031%20topic%20consultation_website%20thumbnail_v1-00-01.pngstatic.ons.gov.uk/images/a891289e-3... https://static.ons.gov.uk/images/a891289e-3abd-4027-bc98-a674d311db56/original/2912%20Census%202031%20topic%20consultation_website%20thumbnail_v1-00-01.png — 200 — 576.7 KiB — 180 ms — h2https://static.ons.gov.uk/images/fb9be62f-1124-43ab-ad1d-21f106e09c4c/original/image%20%289%29.pngstatic.ons.gov.uk/images/fb9be62f-1... https://static.ons.gov.uk/images/fb9be62f-1124-43ab-ad1d-21f106e09c4c/original/image%20%289%29.png — 200 — 6.6 KiB — 159 ms — h2https://static.ons.gov.uk/images/835f0472-877e-4ad2-b120-1416726ea48a/original/Explore-local-statistics_thumbnail-homepage%20%28002%29.pngstatic.ons.gov.uk/images/835f0472-8... https://static.ons.gov.uk/images/835f0472-877e-4ad2-b120-1416726ea48a/original/Explore-local-statistics_thumbnail-homepage%20%28002%29.png — 200 — 49.0 KiB — 178 ms — h2https://static.ons.gov.uk/images/218433dd-b40c-44ce-96fd-b3991931b67c/original/GOV-UK-logo%20%28003%29.pngstatic.ons.gov.uk/images/218433dd-b... https://static.ons.gov.uk/images/218433dd-b40c-44ce-96fd-b3991931b67c/original/GOV-UK-logo%20%28003%29.png — 200 — 40.2 KiB — 107 ms — h2https://cdn.ons.gov.uk/assets/images/logo-ogl-footer.svgcdn.ons.gov.uk/assets/images/logo-o... https://cdn.ons.gov.uk/assets/images/logo-ogl-footer.svg — 200 — 779 B — 31 ms — h2https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.jsajax.googleapis.com/ajax/libs/jquer... https://ajax.googleapis.com/ajax/libs/jquery/3.7.1/jquery.min.js — 200 — 30.4 KiB — 116 ms — h2https://cdn.ons.gov.uk/sixteens/418c927/js/main.jscdn.ons.gov.uk/sixteens/418c927/js/... https://cdn.ons.gov.uk/sixteens/418c927/js/main.js — 200 — 16.8 KiB — 33 ms — h2https://www.ons.gov.uk/js/app.jswww.ons.gov.uk/js/app.js https://www.ons.gov.uk/js/app.js — 200 — 110.3 KiB — 53 ms — h2https://www.googletagmanager.com/gtm.js?id=GTM-MBCBVQSwww.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-MBCBVQS — 200 — 149.5 KiB — 185 ms — h2https://cdn.ons.gov.uk/assets/images/icon-sprite/v2/icon-sprite.svgcdn.ons.gov.uk/assets/images/icon-s... https://cdn.ons.gov.uk/assets/images/icon-sprite/v2/icon-sprite.svg — 200 — 3.9 KiB — 32 ms — h2https://cdn.ons.gov.uk/assets/fonts/open-sans-regular/OpenSans-Regular-webfont.woff2cdn.ons.gov.uk/assets/fonts/open-sa... https://cdn.ons.gov.uk/assets/fonts/open-sans-regular/OpenSans-Regular-webfont.woff2 — 200 — 18.1 KiB — 69 ms — h2https://cdn.ons.gov.uk/assets/fonts/open-sans-bold/OpenSans-Bold-webfont.woff2cdn.ons.gov.uk/assets/fonts/open-sa... https://cdn.ons.gov.uk/assets/fonts/open-sans-bold/OpenSans-Bold-webfont.woff2 — 200 — 17.6 KiB — 70 ms — h2https://static.ons.gov.uk/images/0d844d98-3f70-4c20-9a8b-5c8241aa2c3d/original/Homepage-thumbnail_Navigating-numbers.pngstatic.ons.gov.uk/images/0d844d98-3... https://static.ons.gov.uk/images/0d844d98-3f70-4c20-9a8b-5c8241aa2c3d/original/Homepage-thumbnail_Navigating-numbers.png — 200 — 18.5 KiB — 70 ms — h2https://static.ons.gov.uk/images/49166bf5-1f2a-4e79-9650-bcfafe8cc58c/original/home12345.pngstatic.ons.gov.uk/images/49166bf5-1... https://static.ons.gov.uk/images/49166bf5-1f2a-4e79-9650-bcfafe8cc58c/original/home12345.png — 200 — 5.5 KiB — 70 ms — h2https://www.ons.gov.uk/cdn-cgi/challenge-platform/scripts/jsd/main.jswww.ons.gov.uk/cdn-cgi/challenge-pl... https://www.ons.gov.uk/cdn-cgi/challenge-platform/scripts/jsd/main.js — 302 — 391 B — 30 ms — h2https://www.ons.gov.uk/cdn-cgi/challenge-platform/h/b/scripts/jsd/0b8fb825cb67/main.js?www.ons.gov.uk/cdn-cgi/challenge-pl... https://www.ons.gov.uk/cdn-cgi/challenge-platform/h/b/scripts/jsd/0b8fb825cb67/main.js? — 200 — 12.0 KiB — 22 ms — h2https://www.ons.gov.uk/cdn-cgi/challenge-platform/h/b/jsd/oneshot/0b8fb825cb67/0.7989566937318935:1776802399:zTKHR5rhRQnoOcPGrKEmCIpeWlJ-83Z6Gei8kRIaJDY/9eff3a438d1753c5www.ons.gov.uk/cdn-cgi/challenge-pl... https://www.ons.gov.uk/cdn-cgi/challenge-platform/h/b/jsd/oneshot/0b8fb825cb67/0.7989566937318935:1776802399:zTKHR5rhRQnoOcPGrKEmCIpeWlJ-83Z6Gei8kRIaJDY/9eff3a438d1753c5 — 200 — 973 B — 43 ms — h2https://www.ons.gov.uk/favicon.icowww.ons.gov.uk/favicon.ico https://www.ons.gov.uk/favicon.ico — 200 — 1.4 KiB — 26 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 1337ms · 304KB · £14/mo
Google Tag Manager
Tag Manager
Costly
Execution 440ms
Transfer 149 KB
Unused 34%
Monthly Cost £5/mo
www.ons.gov.uk
Other
Costly
Execution 262ms
Transfer 14 KB
Monthly Cost £3/mo
www.ons.gov.uk
Other
Costly
Execution 245ms
Transfer 0 KB
Monthly Cost £3/mo
Unattributable
Other
Optional
Execution 186ms
Transfer 0 KB
Monthly Cost £2/mo
ajax.googleapis.com
Other
Optional
Execution 122ms
Transfer 30 KB
Monthly Cost £1/mo
www.ons.gov.uk
Other
Optional
Execution 82ms
Transfer 110 KB
Unused 83%
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 440ms and costs ~£5/month
  • www.ons.gov.uk adds 262ms and costs ~£3/month
  • www.ons.gov.uk adds 245ms and costs ~£3/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 440ms 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.ons.gov.uk takes 262ms 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.ons.gov.uk takes 245ms 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

83% of www.ons.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

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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