Performance
· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.Performance-focused optimization plan with CWV assessment, top offenders, and time estimates
First Contentful Paint First Contentful Paint — how long until the browser renders the first piece of content. Under 1.8s is good.
4.36 s
Largest Contentful Paint Largest Contentful Paint — how long until the largest visible element loads. Under 2.5s is good.
9.70 s
Total Blocking Time Total Blocking Time — total time the main thread was blocked, preventing user input. Under 200ms is good.
2.58 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.
6.84 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.
24.15 s
Page Load Progression
FPage Weight BudgetAction3.3 MB transferred, 97 requestsFIX
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
JavaScript bundle is large — code-split routes and lazy-load off-screen components to defer.
Source: web.dev
DThird-Party ImpactAction100% third-party, 0 ms blockingFIX
FJS Execution CostAction5608ms total JS executionFIX
Main Thread Breakdown
| Script | Total | Scripting | Parse/Compile | Party |
|---|---|---|---|---|
| https://www.which.co.uk/static/deliver/static/js/main.caa69650.js | 1299ms | 1147ms | 140ms | 3rd |
| Unattributable | 860ms | 234ms | 0ms | 3rd |
| https://www.which.co.uk/ | 811ms | 26ms | 21ms | 3rd |
| https://www.googletagmanager.com/gtm.js?id=GTM-TKMRQ5B3>m=4e64h1 | 790ms | 718ms | 61ms | 3rd |
| https://www.googletagmanager.com/gtag/js?id=G-B458RCR961&cx=c>m=4e64h1 | 349ms | 271ms | 42ms | 3rd |
| https://www.googletagmanager.com/gtm.js?id=GTM-5MWP3HF | 347ms | 317ms | 27ms | 3rd |
| https://pagead2.googlesyndication.com/pagead/managed/js/gpt/m202604170101/pubads_impl.js?cb=31097963 | 343ms | 291ms | 39ms | 3rd |
| https://cdn-ukwest.onetrust.com/scripttemplates/202604.1.0/otBannerSdk.js | 284ms | 207ms | 42ms | 3rd |
| https://www.googletagmanager.com/gtag/destination?id=AW-973097869&cx=c>m=4e64h1 | 191ms | 162ms | 29ms | 3rd |
| https://www.googletagmanager.com/gtag/destination?id=DC-4401979&cx=c>m=4e64h1 | 158ms | 128ms | 29ms | 3rd |
Showing top 10 of 12 scripts
FJS BundlesAction32 scripts, 837 KB unusedFIX
Downloaded but never executed on this page.
| Script | Size | Unused | Minified | Party |
|---|---|---|---|---|
| https://www.which.co.uk/static/deliver/static/js/main.caa69650.js | 431 KB | 194 KB (45%) | ✓ | 3rd |
| https://www.which.co.uk/static/deliver/static/js/seatbelt.3ff239be.js | 136 KB | 116 KB (85%) | ✓ | 3rd |
| https://pagead2.googlesyndication.com/pagead/managed/js/gpt/m202604170101/pubads_impl.js?cb=31097963 | 187 KB | 109 KB (58%) | ✓ | 3rd |
| https://cdn-ukwest.onetrust.com/scripttemplates/202604.1.0/otBannerSdk.js | 131 KB | 73 KB (56%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtag/destination?id=DC-4401979&cx=c>m=4e64h1 | 131 KB | 65 KB (50%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtag/js?id=G-B458RCR961&cx=c>m=4e64h1 | 168 KB | 65 KB (39%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtag/destination?id=AW-973097869&cx=c>m=4e64h1 | 144 KB | 63 KB (44%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtm.js?id=GTM-5MWP3HF | 118 KB | 60 KB (51%) | ✓ | 3rd |
| https://www.googletagmanager.com/gtm.js?id=GTM-TKMRQ5B3>m=4e64h1 | 207 KB | 52 KB (25%) | ✓ | 3rd |
| https://www.which.co.uk/static/deliver/static/js/libs.d2d8f11b.js | 92 KB | 40 KB (44%) | ✓ | 3rd |
Consider code splitting or tree shaking to reduce unused code.
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.
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.
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.
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.
This bundle has high unused code — code-split or tree-shake to ship only what executes.
Source: web.dev / Lighthouse coverage
FPage Weight InventoryAction97 resources · 3.3 MB · 837 KB savings availableFIX
~837.0 KiB of savings available
| # | Resource | Type | Size | Cache | Unused | 3rd party | Issues |
|---|---|---|---|---|---|---|---|
| — | — | ||||||
| 97 resources | 3.3 MiB | ||||||
FHTTP CachingActionNo cache headersFIX
No Cache-Control header
Adding a Cache-Control header can significantly improve repeat-visit performance.
BImage Optimization29 images, 0 KB saveableREVIEW
Set explicit width and height to prevent CLS.
Performance issues directly impact user engagement and conversion rates.
CGreen HostingActionWhether the site is served from green-energy infrastructureREVIEW
Green Hosting
No green hosting detected
A+Text CompressionAll text resources are compressedPASS
A+Font Loading3 fonts (107 KB)PASS
Web fonts
3
107 KB total
Render-blocking
0
of 3
Dominant font-display
swap
Most common across fonts
Font loading timeline
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 CachingAll resources properly cachedPASS
A+Critical Rendering PathNo render-blocking resourcesPASS
A+Resource Hints12 hints, 0 missing preconnectsPASS
Current Resource Hints
12 resource hints configured
A+Render-Blocking ResourcesNo render-blocking resources detectedPASS
A+Third-Party ResourcesNo third-party resources detectedPASS
Network Waterfall97 requests over 3175msINFO
Third-Party Script CostPer-script blocking time, transfer cost, and cache headersINFO
| Script | Category | Execution | Transfer | Unused | Monthly Cost | Verdict |
|---|---|---|---|---|---|---|
www.which.co.uk www.which.co.uk | Other | 1299ms | 431 KB | 45% | £13/mo | Costly |
Unattributable Unattributable | Other | 860ms | 0 KB | — | £9/mo | Costly |
www.which.co.uk www.which.co.uk | Other | 811ms | 56 KB | — | £8/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 790ms | 207 KB | 25% | £8/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 349ms | 168 KB | 39% | £4/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 347ms | 118 KB | 51% | £4/mo | Costly |
pagead2.googlesyndication.com pagead2.googlesyndication.com | Other | 343ms | 187 KB | 58% | £4/mo | Costly |
cdn-ukwest.onetrust.com cdn-ukwest.onetrust.com | Other | 284ms | 131 KB | 56% | £3/mo | Costly |
Google Tag Manager www.googletagmanager.com | Tag Manager | 191ms | 144 KB | 44% | £2/mo | Optional |
Google Tag Manager www.googletagmanager.com | Tag Manager | 158ms | 131 KB | 50% | £2/mo | Optional |
www.which.co.uk www.which.co.uk | Other | 101ms | 92 KB | 44% | £1/mo | Optional |
pagead2.googlesyndication.com pagead2.googlesyndication.com | Other | 75ms | 34 KB | — | £1/mo | Optional |
These scripts may cost more than they're worth
- www.which.co.uk adds 1299ms and costs ~£13/month
- Unattributable adds 860ms and costs ~£9/month
- www.which.co.uk adds 811ms and costs ~£8/month
- Google Tag Manager adds 790ms and costs ~£8/month
- Google Tag Manager adds 349ms and costs ~£4/month
- Google Tag Manager adds 347ms and costs ~£4/month
- pagead2.googlesyndication.com adds 343ms and costs ~£4/month
- cdn-ukwest.onetrust.com adds 284ms and costs ~£3/month
100% of JavaScript execution time is spent on third-party scripts. Consider auditing which scripts are essential.
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.which.co.uk takes 1299ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Unattributable takes 860ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
www.which.co.uk takes 811ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 790ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 349ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
Google Tag Manager takes 347ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
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.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev
pagead2.googlesyndication.com takes 343ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
58% of pagead2.googlesyndication.com's code is unused. The script may be loading features you don't use.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev
cdn-ukwest.onetrust.com takes 284ms of CPU time. Consider loading it asynchronously or replacing it with a lighter alternative.
This script has high main-thread execution time — optimize hot paths or defer.
Source: web.dev
56% of cdn-ukwest.onetrust.com's code is unused. The script may be loading features you don't use.
Bundle has high unused-code ratio — tree-shaking and route-splitting recover the wasted bytes.
Source: web.dev