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Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
78
GRADE
C
FIX
5
REVIEW
2
PASS
8
INFO
2

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

Mobile Desktop
46
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
97
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.

3.01 s

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

13.53 s

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

772 ms

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

0.052

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

5.51 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.

13.64 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 2 REVIEW 5 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 (1.9 MiB of 1.9 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
D
JS Execution Cost
Action
2463ms total JS execution
FIX
2463ms total JS execution
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-NDH...: 465ms CPU time
Warning::
Unattributable: 343ms CPU time
Warning::
https://www.imperial.ac.uk/assets/website/js/build...: 284ms CPU time
Warning::
https://cdn-ukwest.onetrust.com/scripttemplates/20...: 258ms CPU time
Info::
https://cdn-ukwest.onetrust.com/scripttemplates/ot...: 230ms CPU time
Warning::
Third-party scripts: 2463ms (100% of total)

Main Thread Breakdown

Script Evaluation 55% Other 19% Style & Layout 10% Script Parsing & Compilation 9%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-NDH95W87465ms373ms57ms3rd
Unattributable343ms11ms0ms3rd
https://www.imperial.ac.uk/assets/website/js/build/application.2.3.5.min.js284ms104ms15ms3rd
https://cdn-ukwest.onetrust.com/scripttemplates/202511.1.0/otBannerSdk.js258ms180ms37ms3rd
https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.js230ms211ms4ms3rd
https://cdnjs.cloudflare.com/ajax/libs/gsap/3.10.4/gsap.min.js195ms39ms7ms3rd
https://www.googletagmanager.com/gtag/js?id=G-LME5ZDDFS0&cx=c&gtm=4e64h1168ms130ms37ms3rd
https://www.imperial.ac.uk/165ms6ms1ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-17946844669&cx=c&gtm=4e64h1139ms107ms30ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-11002703160&cx=c&gtm=4e64h1137ms107ms29ms3rd

Showing top 10 of 11 scripts

D
JS Bundles
Action
14 scripts, 367 KB unused
FIX
14 scripts, 367 KB unused
Warning::
https://www.googletagmanager.com/gtag/js?id=G-LME5...: 68 KB unused (40%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-NDH...: 67 KB unused (35%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-179...: 65 KB unused (45%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-110...: 65 KB unused (45%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://cdn-ukwest.onetrust.com/scripttemplates/20...: 64 KB unused (52%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 367 KB
367 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.googletagmanager.com/gtag/js?id=G-LME5ZDDFS0&cx=c&gtm=4e64h1171 KB68 KB (40%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-NDH95W87191 KB67 KB (35%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-17946844669&cx=c&gtm=4e64h1145 KB65 KB (45%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-11002703160&cx=c&gtm=4e64h1146 KB65 KB (45%)✓3rd
https://cdn-ukwest.onetrust.com/scripttemplates/202511.1.0/otBannerSdk.js124 KB64 KB (52%)✓3rd
https://www.imperial.ac.uk/assets/website/js/build/application.2.3.5.min.js52 KB37 KB (72%)✓3rd
https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/6d9efb9d-b605-406f-b8b3-c9644859d064.json2 KB—✓3rd
https://cdnjs.cloudflare.com/ajax/libs/gsap/3.10.4/gsap.min.js23 KB—✓3rd
https://www.imperial.ac.uk/assets/website/js/require/vendor/jquery3-migrate.js15 KB—✗3rd
https://browser.sentry-cdn.com/8.33.0/bundle.min.js24 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

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

D
Page Weight Inventory
Action
41 resources · 1.9 MB · 367 KB savings available
FIX
41 resources · 1.9 MB · 367 KB savings available
Warning::
Page weight 944 KB over 1MB target
Got: 1.9 MB Expected: ≤ 1 MB
Info::
~367 KB of savings available
Warning::
40 third-party resources (100% of weight)
Info::
2 resources over 200KB
41 resources · 1.9 MiB · 40 third-party 943.6 KiB over 1MB target
JavaScript 51% · 1007.3 KiB
Image 37% · 726.9 KiB
Font 6% · 110.8 KiB
CSS 5% · 91.3 KiB
Other 1% · 19.6 KiB
Document 1% · 11.8 KiB

~367.3 KiB of savings available

Remove unused JavaScript: 367.3 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.

C
Page Weight Budget
Action
1.9 MB transferred, 41 requests
REVIEW
1.9 MB transferred, 41 requests
Info::
Page weighs 5.0 MB (1.9 MB transferred)
Warning::
JavaScript is 935 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 935 KB
Info::
41 HTTP requests
Info::
Estimated 0.40 g CO2 per page load
1.9 MB / 2.4 MB budget
1.9 MB transferred
41 requests
0.40 g CO2 per page load
JavaScript 935.2 KiB 48%
Images 717.2 KiB 36%
Fonts 110.8 KiB 6%
Other 94.3 KiB 5%
CSS 86.7 KiB 4%
HTML 11.3 KiB 1%
Other 9.7 KiB 0%
Other 2.5 KiB 0%
0 2.4 MB 4.9 MB
Approaching limit

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

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
Image Optimization
8 images, 0 KB saveable
PASS
8 images, 0 KB saveable
Warning::
https://pxl01-imperialacuk.terminalfour.net/fit-in... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
8 images 717 KB
0 oversized -0 KB
2 legacy format
1 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

A+
Font Loading
2 fonts (111 KB)
PASS
2 fonts (111 KB)
Info::
2 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

2

111 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 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s 14.0 s FCP 3.0 s LCP 13.5 shttps://www.imperial.ac.uk/assets/website/fonts/icons/fonts/imperial-icons.woff?vrmf2dimperial-icons.woff https://www.imperial.ac.uk/assets/website/fonts/icons/fonts/imperial-icons.woff?vrmf2d — 12.4 KiB — 36 ms — font-display: swaphttps://www.imperial.ac.uk/assets/website/fonts/imperial-sans/ImperialText-VF.woff2ImperialText-VF.woff2 https://www.imperial.ac.uk/assets/website/fonts/imperial-sans/ImperialText-VF.woff2 — 98.4 KiB — 182 ms — font-display: swap
imperial-icons.woff woff swap
Size 12 KB
Load time 36 ms
Start 220 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
ImperialText-VF.woff2 woff2 swap
Size 98 KB
Load time 182 ms
Start 220 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)

Fixing the unchecked items could save ~4 KB and ~8 ms

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
41 requests over 1376ms
INFO
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s 14.0 s FCP 3.0 s LCP 13.5 shttps://imperial.ac.uk// https://imperial.ac.uk/ — 301 — 548 B — 92 ms — http/1.1https://www.imperial.ac.uk/www.imperial.ac.uk/ https://www.imperial.ac.uk/ — 200 — 11.3 KiB — 116 ms — http/1.1https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/OtAutoBlock.jscdn-ukwest.onetrust.com/consent/6d9... https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/OtAutoBlock.js — 200 — 3.4 KiB — 148 ms — h2https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.jscdn-ukwest.onetrust.com/scripttempl... https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.js — 200 — 9.1 KiB — 124 ms — h2https://www.imperial.ac.uk/assets/website/fonts/icons/fonts/imperial-icons.woff?vrmf2dwww.imperial.ac.uk/assets/website/f... https://www.imperial.ac.uk/assets/website/fonts/icons/fonts/imperial-icons.woff?vrmf2d — 200 — 12.4 KiB — 36 ms — http/1.1https://www.imperial.ac.uk/assets/website/fonts/imperial-sans/ImperialText-VF.woff2www.imperial.ac.uk/assets/website/f... https://www.imperial.ac.uk/assets/website/fonts/imperial-sans/ImperialText-VF.woff2 — 200 — 98.4 KiB — 182 ms — http/1.1https://www.imperial.ac.uk/assets/website/stylesheets/css/screen.2.3.5.csswww.imperial.ac.uk/assets/website/s... https://www.imperial.ac.uk/assets/website/stylesheets/css/screen.2.3.5.css — 200 — 84.3 KiB — 126 ms — http/1.1https://www.imperial.ac.uk/assets/website/js/lib/jquery3.min.jswww.imperial.ac.uk/assets/website/j... https://www.imperial.ac.uk/assets/website/js/lib/jquery3.min.js — 200 — 30.5 KiB — 119 ms — http/1.1https://pxl01-imperialacuk.terminalfour.net/fit-in/1350x1350/filters:format(webp)/prod01/channel_3/media/images/landscape-3000X2000/260420_queens_tower_restoration.jpgpxl01-imperialacuk.terminalfour.net... https://pxl01-imperialacuk.terminalfour.net/fit-in/1350x1350/filters:format(webp)/prod01/channel_3/media/images/landscape-3000X2000/260420_queens_tower_restoration.jpg — 200 — 217.9 KiB — 55 ms — h2https://www.imperial.ac.uk/assets/website/js/build/application.2.3.5.min.jswww.imperial.ac.uk/assets/website/j... https://www.imperial.ac.uk/assets/website/js/build/application.2.3.5.min.js — 200 — 51.9 KiB — 65 ms — http/1.1https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/6d9efb9d-b605-406f-b8b3-c9644859d064.jsoncdn-ukwest.onetrust.com/consent/6d9... https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/6d9efb9d-b605-406f-b8b3-c9644859d064.json — 200 — 2.2 KiB — 107 ms — h2https://cdnjs.cloudflare.com/ajax/libs/gsap/3.10.4/gsap.min.jscdnjs.cloudflare.com/ajax/libs/gsap... https://cdnjs.cloudflare.com/ajax/libs/gsap/3.10.4/gsap.min.js — 200 — 23.4 KiB — 91 ms — h2https://www.imperial.ac.uk/assets/website/stylesheets/css/print.2.3.5.csswww.imperial.ac.uk/assets/website/s... https://www.imperial.ac.uk/assets/website/stylesheets/css/print.2.3.5.css — 200 — 2.4 KiB — 33 ms — http/1.1https://www.googletagmanager.com/gtm.js?id=GTM-NDH95W87www.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-NDH95W87 — 200 — 191.1 KiB — 149 ms — h2https://www.imperial.ac.uk/assets/website/images/svg/icons.svgwww.imperial.ac.uk/assets/website/i... https://www.imperial.ac.uk/assets/website/images/svg/icons.svg — 200 — 6.4 KiB — 34 ms — http/1.1data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' f…www.w3.org/2000/svg' width='24' hei... data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' width='24' height='24' f… — 200 — 0 B — 0 ms — datahttps://geolocation.onetrust.com/cookieconsentpub/v1/geo/locationgeolocation.onetrust.com/cookiecons... https://geolocation.onetrust.com/cookieconsentpub/v1/geo/location — 200 — 225 B — 68 ms — h2https://www.imperial.ac.uk/assets/website/js/require/vendor/jquery3-migrate.jswww.imperial.ac.uk/assets/website/j... https://www.imperial.ac.uk/assets/website/js/require/vendor/jquery3-migrate.js — 200 — 15.1 KiB — 34 ms — http/1.1data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' width='20' height='20' f…www.w3.org/2000/svg' width='20' hei... data:image/svg+xml;charset=utf-8,%3Csvg xmlns='http://www.w3.org/2000/svg' width='20' height='20' f… — 200 — 0 B — 0 ms — datahttps://browser.sentry-cdn.com/8.33.0/bundle.min.jsbrowser.sentry-cdn.com/8.33.0/bundl... https://browser.sentry-cdn.com/8.33.0/bundle.min.js — 200 — 24.4 KiB — 24 ms — h2https://www.googletagmanager.com/gtag/js?id=AW-11002703160&cx=c&gtm=4e64h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=AW-11002703160&cx=c&gtm=4e64h1 — 200 — 145.5 KiB — 40 ms — h2https://www.googletagmanager.com/gtag/js?id=AW-17946844669&cx=c&gtm=4e64h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=AW-17946844669&cx=c&gtm=4e64h1 — 200 — 145.1 KiB — 58 ms — h2https://www.googletagmanager.com/gtag/js?id=G-LME5ZDDFS0&cx=c&gtm=4e64h1www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=G-LME5ZDDFS0&cx=c&gtm=4e64h1 — 200 — 171.3 KiB — 60 ms — h2https://cdn-ukwest.onetrust.com/scripttemplates/202511.1.0/otBannerSdk.jscdn-ukwest.onetrust.com/scripttempl... https://cdn-ukwest.onetrust.com/scripttemplates/202511.1.0/otBannerSdk.js — 200 — 124.4 KiB — 66 ms — h2https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/019a0b3b-cce4-7123-ac41-14d6bf474427/en.jsoncdn-ukwest.onetrust.com/consent/6d9... https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/019a0b3b-cce4-7123-ac41-14d6bf474427/en.json — 200 — 69.8 KiB — 88 ms — h2https://www.imperial.ac.uk/assets/website/images/favicon/favicon.icowww.imperial.ac.uk/assets/website/i... https://www.imperial.ac.uk/assets/website/images/favicon/favicon.ico — 200 — 1.8 KiB — 34 ms — http/1.1https://www.imperial.ac.uk/assets/website/images/favicon/favicon.svgwww.imperial.ac.uk/assets/website/i... https://www.imperial.ac.uk/assets/website/images/favicon/favicon.svg — 200 — 1014 B — 33 ms — http/1.1https://region1.google-analytics.com/g/collect?v=2&tid=G-LME5ZDDFS0&gtm=45je64h1v885699965z89184169574za20gzb9184169574zd9184169574&_p=1776803868548&gcs=G100&gcd=13q3qPq2q5l1&npa=1&dma_cps=-&dma=1&_eu=EAAAAGA&are=1&cid=1147081870.1776803869&frm=0&ir=1&pscdl=denied&rcb=7&sr=412x823&uaa=&uab=64&uafvl=Chromium%3B147.0.7727.101%7CNot.A%252FBrand%3B8.0.0.0&uam=moto%20g%20power%20(2022)&uamb=1&uap=Android&uapv=11.0&uaw=0&ul=en-us&_s=1&tag_exp=0~115616985~115938466~115938468~117266401~118463261&sid=1776803868&sct=1&seg=0&dl=https%3A%2F%2Fwww.imperial.ac.uk%2F&dt=Imperial%20College%20London&_tu=CA&en=page_view&_fv=1&_nsi=1&_ss=1&ep.fragment=&tfd=944region1.google-analytics.com/g/coll... https://region1.google-analytics.com/g/collect?v=2&tid=G-LME5ZDDFS0&gtm=45je64h1v885699965z89184169574za20gzb9184169574zd9184169574&_p=1776803868548&gcs=G100&gcd=13q3qPq2q5l1&npa=1&dma_cps=-&dma=1&_eu=EAAAAGA&are=1&cid=1147081870.1776803869&frm=0&ir=1&pscdl=denied&rcb=7&sr=412x823&uaa=&uab=64&uafvl=Chromium%3B147.0.7727.101%7CNot.A%252FBrand%3B8.0.0.0&uam=moto%20g%20power%20(2022)&uamb=1&uap=Android&uapv=11.0&uaw=0&ul=en-us&_s=1&tag_exp=0~115616985~115938466~115938468~117266401~118463261&sid=1776803868&sct=1&seg=0&dl=https%3A%2F%2Fwww.imperial.ac.uk%2F&dt=Imperial%20College%20London&_tu=CA&en=page_view&_fv=1&_nsi=1&_ss=1&ep.fragment=&tfd=944 — 204 — 551 B — 107 ms — h2https://cdn-ukwest.onetrust.com/scripttemplates/202511.1.0/assets/otFlat.jsoncdn-ukwest.onetrust.com/scripttempl... https://cdn-ukwest.onetrust.com/scripttemplates/202511.1.0/assets/otFlat.json — 200 — 3.4 KiB — 42 ms — 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0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s 14.0 s FCP 3.0 s LCP 13.5 shttps://imperial.ac.uk// https://imperial.ac.uk/ — 301 — 548 B — 92 ms — http/1.1https://www.imperial.ac.uk/www.imperial.ac.uk/ https://www.imperial.ac.uk/ — 200 — 11.3 KiB — 116 ms — http/1.1https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/OtAutoBlock.jscdn-ukwest.onetrust.com/consent/6d9... https://cdn-ukwest.onetrust.com/consent/6d9efb9d-b605-406f-b8b3-c9644859d064/OtAutoBlock.js — 200 — 3.4 KiB — 148 ms — h2https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.jscdn-ukwest.onetrust.com/scripttempl... https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.js — 200 — 9.1 KiB — 124 ms — h2https://www.imperial.ac.uk/assets/website/fonts/icons/fonts/imperial-icons.woff?vrmf2dwww.imperial.ac.uk/assets/website/f... https://www.imperial.ac.uk/assets/website/fonts/icons/fonts/imperial-icons.woff?vrmf2d — 200 — 12.4 KiB — 36 ms — 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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 2463ms · 904KB · £26/mo
Google Tag Manager
Tag Manager
Costly
Execution 465ms
Transfer 191 KB
Unused 35%
Monthly Cost £5/mo
Unattributable
Other
Costly
Execution 343ms
Transfer 0 KB
Monthly Cost £4/mo
www.imperial.ac.uk
Other
Costly
Execution 284ms
Transfer 52 KB
Unused 72%
Monthly Cost £3/mo
cdn-ukwest.onetrust.com
Other
Costly
Execution 258ms
Transfer 124 KB
Unused 52%
Monthly Cost £3/mo
cdn-ukwest.onetrust.com
Other
Costly
Execution 230ms
Transfer 9 KB
Monthly Cost £2/mo
cdnjs
CDN
Optional
Execution 195ms
Transfer 23 KB
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Optional
Execution 168ms
Transfer 171 KB
Unused 40%
Monthly Cost £2/mo
www.imperial.ac.uk
Other
Optional
Execution 165ms
Transfer 11 KB
Monthly Cost £2/mo
Google Tag Manager
Tag Manager
Optional
Execution 139ms
Transfer 145 KB
Unused 45%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 137ms
Transfer 146 KB
Unused 45%
Monthly Cost £1/mo
www.imperial.ac.uk
Other
Optional
Execution 79ms
Transfer 31 KB
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 465ms and costs ~£5/month
  • Unattributable adds 343ms and costs ~£4/month
  • www.imperial.ac.uk adds 284ms and costs ~£3/month
  • cdn-ukwest.onetrust.com adds 258ms and costs ~£3/month
  • cdn-ukwest.onetrust.com adds 230ms 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

Google Tag Manager takes 465ms 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 343ms 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.imperial.ac.uk takes 284ms 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

72% of www.imperial.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

cdn-ukwest.onetrust.com takes 258ms 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

52% of cdn-ukwest.onetrust.com'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

cdn-ukwest.onetrust.com takes 230ms 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

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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