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https://www.nhs.uk

Performance

· 16 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
93
GRADE
A
FIX
1
REVIEW
2
PASS
11
INFO
2

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

Mobile Desktop
59
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.

1.87 s

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

4.52 s

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

156 ms

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

0.495

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

3.28 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.

4.52 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
16
11 PASS 2 REVIEW 1 FIX
D
Third-Party Impact
Action
87% third-party, 0 ms blocking
FIX
87% third-party, 0 ms blocking
Warning::
Third-party code accounts for 87% of page weight (538.7 KiB of 622.7 KiB)
Info::
Third-party blocking time is low (0 ms)
87%
First-party Third-party
B
JS Execution Cost
705ms total JS execution
REVIEW
705ms total JS execution
Info::
https://www.nhs.uk/static/nhsuk/js/main.bd1962aa3f...: 207ms CPU time
Info::
Unattributable: 173ms CPU time
Info::
https://www.nhs.uk/: 166ms CPU time
Info::
https://assets.nhs.uk/scripts/cookie-consent.js: 159ms CPU time
Info::
Third-party scripts: 332ms (47% of total)

Main Thread Breakdown

Script Evaluation 40% Other 29% Style & Layout 16% Script Parsing & Compilation 7% Rendering 5% Parse HTML & CSS 5%
ScriptTotalScriptingParse/CompileParty
https://www.nhs.uk/static/nhsuk/js/main.bd1962aa3fa4.js207ms144ms30ms1st
Unattributable173ms6ms0ms3rd
https://www.nhs.uk/166ms8ms4ms1st
https://assets.nhs.uk/scripts/cookie-consent.js159ms140ms15ms3rd
C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

A+
Page Weight Budget
623 KB transferred, 13 requests
PASS
623 KB transferred, 13 requests
Info::
Page weighs 1.0 MB (623 KB transferred)
Info::
13 HTTP requests
Info::
Estimated 0.13 g CO2 per page load
623 KB / 2.4 MB budget
623 KB transferred
13 requests
0.13 g CO2 per page load
Images 464.7 KiB 75%
JavaScript 85.5 KiB 14%
Fonts 34.7 KiB 6%
CSS 20.4 KiB 3%
Other 7.1 KiB 1%
HTML 7.0 KiB 1%
Other 3.3 KiB 1%
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+
Image Optimization
1 images, 0 KB saveable
PASS
1 images, 0 KB saveable
Info::
All images are well-optimized
1 images 465 KB
0 oversized -0 KB
1 legacy format
0 missing dimensions CLS risk
A+
Font Loading
2 fonts (35 KB)
PASS
2 fonts (35 KB)
Info::
2 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

2

35 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 FCP 1.9 s LCP 4.5 shttps://assets.nhs.uk/fonts/FrutigerLTW01-55Roman.woff2FrutigerLTW01-55Roman.w... https://assets.nhs.uk/fonts/FrutigerLTW01-55Roman.woff2 — 17.4 KiB — 0 ms — font-display: swaphttps://assets.nhs.uk/fonts/FrutigerLTW01-65Bold.woff2FrutigerLTW01-65Bold.woff2 https://assets.nhs.uk/fonts/FrutigerLTW01-65Bold.woff2 — 17.3 KiB — 0 ms — font-display: swap
FrutigerLTW01-55Roman.w... woff2 swap
Size 17 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
FrutigerLTW01-65Bold.woff2 woff2 swap
Size 17 KB
Load time 0 ms
Start 0 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+
JS Bundles
4 scripts, 0 KB unused
PASS
4 scripts, 0 KB unused
0 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.nhs.uk/static/nhsuk/js/main.bd1962aa3fa4.js46 KB—✓1st
https://www.nhsapp.service.nhs.uk/js/v2/nhsapp.js1 KB—✓3rd
https://assets.nhs.uk/scripts/cookie-consent.js36 KB—✓3rd
https://assets.nhs.uk/scripts/login.js2 KB—✓3rd
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
1
preload
2
dns-prefetch
—
prefetch
—

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
HTTP Caching
max-age=5 (5 seconds)
PASS
max-age=5 (5 seconds)
Info::
Cache-Control header is set
Got: max-age=5
Info::
Effective TTL: 5 seconds (max-age=5)
Info::
No ETag or Last-Modified header
Conditional requests (304 Not Modified) are not possible without validators.
Cache-Control

max-age=5

DirectiveValueMeaning
max-age5Cache for 5 seconds
Effective TTL: 5 seconds
Network Waterfall
13 requests over 0ms
INFO

Performance analysis not available for this result.

Third-Party Script Cost
Per-script blocking time, transfer cost, and cache headers
INFO
47% of JavaScript execution is third-party
First-party Third-party 332ms · 36KB · £3/mo
Unattributable
Other
Optional
Execution 173ms
Transfer 0 KB
Monthly Cost £2/mo
assets.nhs.uk
Other
Optional
Execution 159ms
Transfer 36 KB
Monthly Cost £2/mo

47% of JavaScript execution time comes from third-party scripts.

Why this matters

Third-party scripts (analytics, ads, social, A/B testing) often dominate execution time — every one is a perf-and-privacy tax.

Learn more ▾

Each third-party script is a black box: you don't control when it loads, what it executes, or how much it grows. They often account for a major share of total blocking time on average sites (HTTP Archive's Web Almanac documents the trend). Audit which ones you actually need, defer the rest, and use facade patterns (lite-youtube, lite-vimeo) for embedded media.

Source: web.dev / HTTP Archive Web Almanac

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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