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Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
81
GRADE
B
FIX
3
REVIEW
5
PASS
7
INFO
2

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

Mobile Desktop
55
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
94
Accessibility Measures how accessible the page is for users with disabilities. Checks color contrast, ARIA labels, and semantic HTML.
81
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
66
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.

3.93 s

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

6.99 s

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

486 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.

3.93 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.

8.04 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 5 REVIEW 3 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
D
JS Execution Cost
Action
2006ms total JS execution
FIX
2006ms total JS execution
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-W5R...: 765ms CPU time
Warning::
https://www.kcl.ac.uk/: 363ms CPU time
Warning::
Unattributable: 254ms CPU time
Info::
https://www.kcl.ac.uk/SiteElements/2017/js/jquery-...: 233ms CPU time
Info::
https://www.googletagmanager.com/gtag/destination?...: 223ms CPU time
Warning::
Third-party scripts: 2006ms (100% of total)

Main Thread Breakdown

Script Evaluation 54% Other 15% Style & Layout 15% Script Parsing & Compilation 10%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-W5RSV5765ms654ms81ms3rd
https://www.kcl.ac.uk/363ms9ms2ms3rd
Unattributable254ms8ms0ms3rd
https://www.kcl.ac.uk/SiteElements/2017/js/jquery-3.2.1.min.js?version=7389704&build=171169233ms146ms8ms3rd
https://www.googletagmanager.com/gtag/destination?id=G-1X78DS0GQQ&cx=c&gtm=4e64h1223ms173ms49ms3rd
https://cdn.cookielaw.org/scripttemplates/202503.1.0/otBannerSdk.js167ms83ms54ms3rd
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, 45 requests
REVIEW
1.3 MB transferred, 45 requests
Info::
Page weighs 3.3 MB (1.3 MB transferred)
Warning::
JavaScript is 593 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 593 KB
Info::
45 HTTP requests
Info::
Estimated 0.26 g CO2 per page load
1.3 MB / 2.4 MB budget
1.3 MB transferred
45 requests
0.26 g CO2 per page load
JavaScript 592.8 KiB 46%
Images 448.5 KiB 35%
Fonts 111.8 KiB 9%
CSS 74.6 KiB 6%
Other 26.8 KiB 2%
HTML 18.7 KiB 1%
Other 4.8 KiB 0%
Other 4.2 KiB 0%
0 2.4 MB 4.9 MB
Under budget

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
Image Optimization
Action
18 images, 0 KB saveable
REVIEW
18 images, 0 KB saveable
Warning::
https://www.kcl.ac.uk/image-library/press-photos/b... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/newimages/csss/website-image... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/newimages/research/homepage/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/SiteElements/2017/images/und... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/departmentalimages/research/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/SiteElements/2017/images/kcl... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/SiteElements/2017/images/pos... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/SiteElements/2017/images/int... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/newimages/strategy-2030/stra... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.kcl.ac.uk/SiteElements/2017/images/lap... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
18 images 448 KB
0 oversized -0 KB
5 legacy format
10 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

C
JS Bundles
Action
10 scripts, 199 KB unused
REVIEW
10 scripts, 199 KB unused
Warning::
https://cdn.cookielaw.org/scripttemplates/202503.1...: 70 KB unused (60%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/destination?...: 67 KB unused (39%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-W5R...: 63 KB unused (33%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 199 KB
199 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://cdn.cookielaw.org/scripttemplates/202503.1.0/otBannerSdk.js116 KB70 KB (60%)✓3rd
https://www.googletagmanager.com/gtag/destination?id=G-1X78DS0GQQ&cx=c&gtm=4e64h1170 KB67 KB (39%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-W5RSV5192 KB63 KB (33%)✓3rd
https://www.kcl.ac.uk/SiteElements/2017/js/jquery-3.2.1.min.js?version=7389704&build=17116931 KB—✓3rd
https://cdn.cookielaw.org/consent/167b1744-c18f-47c4-bd0c-e8420eacd9e1/OtAutoBlock.js14 KB—✓3rd
https://cdn.cookielaw.org/scripttemplates/otSDKStub.js9 KB—✓3rd
https://www.kcl.ac.uk/siteelements/2017/js/glide.min.js8 KB—✓3rd
https://www.kcl.ac.uk/SiteElements/2017/js/popper.min.js?version=7468037&build=1711698 KB—✓3rd
https://www.kcl.ac.uk/SiteElements/2017/js/scripts.min.js?version=7806953&build=17116918 KB—✓3rd
https://platform.twitter.com/widgets.js27 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

C
Page Weight Inventory
Action
45 resources · 1.3 MB · 199 KB savings available
REVIEW
45 resources · 1.3 MB · 199 KB savings available
Warning::
Page weight 258 KB over 1MB target
Got: 1.3 MB Expected: ≤ 1 MB
Info::
~199 KB of savings available
Warning::
44 third-party resources (100% of weight)
45 resources · 1.3 MiB · 44 third-party 258.1 KiB over 1MB target
JavaScript 46% · 592.8 KiB
Image 35% · 453.0 KiB
Font 9% · 111.8 KiB
CSS 6% · 79.0 KiB
Other 2% · 26.7 KiB
Document 1% · 18.9 KiB

~199.0 KiB of savings available

Remove unused JavaScript: 199.0 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
4 fonts (112 KB)
PASS
4 fonts (112 KB)
Info::
4 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

4

112 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 5.0 s 6.0 s 7.0 s FCP 3.9 s LCP 7.0 shttps://www.kcl.ac.uk/SiteElements/2017/fonts/KingsBureauGrotesqueFiveOne-woff.woffKingsBureauGrotesqueFiv... https://www.kcl.ac.uk/SiteElements/2017/fonts/KingsBureauGrotesqueFiveOne-woff.woff — 34.8 KiB — 57 ms — font-display: swaphttps://www.kcl.ac.uk/SiteElements/2017/fonts/KingsBureauGrotesqueThreeSeven-woff.woffKingsBureauGrotesqueThr... https://www.kcl.ac.uk/SiteElements/2017/fonts/KingsBureauGrotesqueThreeSeven-woff.woff — 30.0 KiB — 51 ms — font-display: swaphttps://www.kcl.ac.uk/SiteElements/2017/fonts/kcl-icons-woff2.woff2?36ubotkcl-icons-woff2.woff2 https://www.kcl.ac.uk/SiteElements/2017/fonts/kcl-icons-woff2.woff2?36ubot — 29.8 KiB — 49 ms — font-display: swaphttps://www.kcl.ac.uk/SiteElements/2017/fonts/kcl-icons-ttf.ttf?36ubotkcl-icons-ttf.ttf https://www.kcl.ac.uk/SiteElements/2017/fonts/kcl-icons-ttf.ttf?36ubot — 17.2 KiB — 36 ms — font-display: swap
KingsBureauGrotesqueFiv... woff swap
Size 35 KB
Load time 57 ms
Start 360 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
KingsBureauGrotesqueThr... woff swap
Size 30 KB
Load time 51 ms
Start 406 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
kcl-icons-woff2.woff2 woff2 swap
Size 30 KB
Load time 49 ms
Start 411 ms
Risk FOUT — text flashes from fallback to web font
kcl-icons-ttf.ttf ttf swap
Size 17 KB
Load time 36 ms
Start 484 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
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 ~25 KB and ~50 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
45 requests over 972ms
INFO
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s 7.0 s FCP 3.9 s LCP 7.0 shttps://kcl.ac.uk// https://kcl.ac.uk/ — 301 — 243 B — 93 ms — http/1.1http://www.kcl.ac.uk/www.kcl.ac.uk/ http://www.kcl.ac.uk/ — 307 — 0 B — 1 ms — http/1.1https://www.kcl.ac.uk/www.kcl.ac.uk/ https://www.kcl.ac.uk/ — 200 — 18.7 KiB — 136 ms — h2https://www.kcl.ac.uk/siteelements/2017/css/style.min.css?version=8013490?version=8013490www.kcl.ac.uk/siteelements/2017/css... https://www.kcl.ac.uk/siteelements/2017/css/style.min.css?version=8013490?version=8013490 — 200 — 74.6 KiB — 74 ms — h2https://www.kcl.ac.uk/SiteElements/2017/js/jquery-3.2.1.min.js?version=7389704&build=171169www.kcl.ac.uk/SiteElements/2017/js/... https://www.kcl.ac.uk/SiteElements/2017/js/jquery-3.2.1.min.js?version=7389704&build=171169 — 200 — 30.6 KiB — 72 ms — h2https://cdn.cookielaw.org/consent/167b1744-c18f-47c4-bd0c-e8420eacd9e1/OtAutoBlock.jscdn.cookielaw.org/consent/167b1744-... https://cdn.cookielaw.org/consent/167b1744-c18f-47c4-bd0c-e8420eacd9e1/OtAutoBlock.js — 200 — 14.5 KiB — 117 ms — h2https://cdn.cookielaw.org/scripttemplates/otSDKStub.jscdn.cookielaw.org/scripttemplates/o... https://cdn.cookielaw.org/scripttemplates/otSDKStub.js — 200 — 9.1 KiB — 102 ms — h2https://www.kcl.ac.uk/siteelements/2017/js/glide.min.jswww.kcl.ac.uk/siteelements/2017/js/... https://www.kcl.ac.uk/siteelements/2017/js/glide.min.js — 200 — 7.8 KiB — 62 ms — h2https://www.kcl.ac.uk/SiteElements/2017/images/undergraduate.pngwww.kcl.ac.uk/SiteElements/2017/ima... https://www.kcl.ac.uk/SiteElements/2017/images/undergraduate.png — 200 — 7.6 KiB — 83 ms — h2https://www.kcl.ac.uk/SiteElements/2017/images/postgraduate.pngwww.kcl.ac.uk/SiteElements/2017/ima... https://www.kcl.ac.uk/SiteElements/2017/images/postgraduate.png — 200 — 8.3 KiB — 81 ms — h2https://www.kcl.ac.uk/SiteElements/2017/images/laptop-thin-kcl-1.pngwww.kcl.ac.uk/SiteElements/2017/ima... https://www.kcl.ac.uk/SiteElements/2017/images/laptop-thin-kcl-1.png — 200 — 8.2 KiB — 64 ms — h2https://www.kcl.ac.uk/SiteElements/2017/images/international.pngwww.kcl.ac.uk/SiteElements/2017/ima... https://www.kcl.ac.uk/SiteElements/2017/images/international.png — 200 — 11.6 KiB — 65 ms — h2https://www.kcl.ac.uk/image-library/press-photos/bush-house-view.jpg?f=webpwww.kcl.ac.uk/image-library/press-p... https://www.kcl.ac.uk/image-library/press-photos/bush-house-view.jpg?f=webp — 200 — 35.5 KiB — 71 ms — h2https://www.kcl.ac.uk/SiteElements/2017/js/popper.min.js?version=7468037&build=171169www.kcl.ac.uk/SiteElements/2017/js/... https://www.kcl.ac.uk/SiteElements/2017/js/popper.min.js?version=7468037&build=171169 — 200 — 7.9 KiB — 67 ms — h2https://www.kcl.ac.uk/SiteElements/2017/js/scripts.min.js?version=7806953&build=171169www.kcl.ac.uk/SiteElements/2017/js/... https://www.kcl.ac.uk/SiteElements/2017/js/scripts.min.js?version=7806953&build=171169 — 200 — 18.1 KiB — 72 ms — h2https://www.kcl.ac.uk/newimages/strategy-2030/strategy-2030-hero-banner-robot-and-campus-cropped-368x208.jpg?f=webpwww.kcl.ac.uk/newimages/strategy-20... https://www.kcl.ac.uk/newimages/strategy-2030/strategy-2030-hero-banner-robot-and-campus-cropped-368x208.jpg?f=webp — 200 — 11.4 KiB — 69 ms — h2https://www.kcl.ac.uk/newimages/csss/website-images/research-2-Cropped-368x208.jpg?f=webpwww.kcl.ac.uk/newimages/csss/websit... https://www.kcl.ac.uk/newimages/csss/website-images/research-2-Cropped-368x208.jpg?f=webp — 200 — 18.0 KiB — 70 ms — h2https://www.kcl.ac.uk/newimages/research/homepage/guys-cancer-centre-immunology-lab-promo-image-cropped-368x208.png?f=webpwww.kcl.ac.uk/newimages/research/ho... https://www.kcl.ac.uk/newimages/research/homepage/guys-cancer-centre-immunology-lab-promo-image-cropped-368x208.png?f=webp — 200 — 7.6 KiB — 72 ms — h2https://www.kcl.ac.uk/departmentalimages/research/kings-doctoral-college-induction-event-2025-cropped-368x208.jpg?f=webpwww.kcl.ac.uk/departmentalimages/re... https://www.kcl.ac.uk/departmentalimages/research/kings-doctoral-college-induction-event-2025-cropped-368x208.jpg?f=webp — 200 — 17.6 KiB — 69 ms — h2https://www.kcl.ac.uk/newimages/nmpc/calories.x84e42797.png?f=webpwww.kcl.ac.uk/newimages/nmpc/calori... https://www.kcl.ac.uk/newimages/nmpc/calories.x84e42797.png?f=webp — 200 — 20.7 KiB — 93 ms — h2https://www.kcl.ac.uk/newimages/folsm/hero-banner/bmeis/bmeis-surgery.x0645e1a5.png?f=webpwww.kcl.ac.uk/newimages/folsm/hero-... https://www.kcl.ac.uk/newimages/folsm/hero-banner/bmeis/bmeis-surgery.x0645e1a5.png?f=webp — 200 — 29.0 KiB — 93 ms — h2https://www.kcl.ac.uk/newimages/sspp/ecs/events/justed.x805c9612.jpeg?w=1285&h=476&crop=780,440,194,18&f=webpwww.kcl.ac.uk/newimages/sspp/ecs/ev... https://www.kcl.ac.uk/newimages/sspp/ecs/events/justed.x805c9612.jpeg?w=1285&h=476&crop=780,440,194,18&f=webp — 200 — 20.0 KiB — 91 ms — h2https://www.kcl.ac.uk/newimages/ai/780x440/jimmy-wales-ai-leaders-event-780x440.x5d1c74dc.png?w=781&h=440&crop=780,440,1,0&f=webpwww.kcl.ac.uk/newimages/ai/780x440/... https://www.kcl.ac.uk/newimages/ai/780x440/jimmy-wales-ai-leaders-event-780x440.x5d1c74dc.png?w=781&h=440&crop=780,440,1,0&f=webp — 200 — 23.4 KiB — 91 ms — h2https://www.kcl.ac.uk/newimages/law/event-promos-780x440/book-launch-a-research-agenda-for-law-finance-and-the-environment-tn.x6d4b47ff.png?f=webpwww.kcl.ac.uk/newimages/law/event-p... https://www.kcl.ac.uk/newimages/law/event-promos-780x440/book-launch-a-research-agenda-for-law-finance-and-the-environment-tn.x6d4b47ff.png?f=webp — 200 — 25.3 KiB — 91 ms — h2https://www.kcl.ac.uk/SiteElements/2017/images/kcl-logo.svgwww.kcl.ac.uk/SiteElements/2017/ima... https://www.kcl.ac.uk/SiteElements/2017/images/kcl-logo.svg — 200 — 4.5 KiB — 93 ms — h2https://platform.twitter.com/widgets.jsplatform.twitter.com/widgets.js https://platform.twitter.com/widgets.js — 200 — 27.3 KiB — 119 ms — h2https://www.googletagmanager.com/gtm.js?id=GTM-W5RSV5www.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-W5RSV5 — 200 — 192.0 KiB — 171 ms — h2https://cdn.cookielaw.org/consent/167b1744-c18f-47c4-bd0c-e8420eacd9e1/167b1744-c18f-47c4-bd0c-e8420eacd9e1.jsoncdn.cookielaw.org/consent/167b1744-... https://cdn.cookielaw.org/consent/167b1744-c18f-47c4-bd0c-e8420eacd9e1/167b1744-c18f-47c4-bd0c-e8420eacd9e1.json — 200 — 2.5 KiB — 84 ms — h2https://www.kcl.ac.uk/SiteElements/2017/fonts/KingsBureauGrotesqueFiveOne-woff.woffwww.kcl.ac.uk/SiteElements/2017/fon... https://www.kcl.ac.uk/SiteElements/2017/fonts/KingsBureauGrotesqueFiveOne-woff.woff — 200 — 34.8 KiB — 57 ms — h2https://www.kcl.ac.uk/newimages/ah/shakespeare-flat-hero.xcf32bb85.png?f=webpwww.kcl.ac.uk/newimages/ah/shakespe... https://www.kcl.ac.uk/newimages/ah/shakespeare-flat-hero.xcf32bb85.png?f=webp — 200 — 84.1 KiB — 70 ms — h2
0 ms 1.0 s 2.0 s 3.0 s 4.0 s 5.0 s 6.0 s 7.0 s FCP 3.9 s LCP 7.0 shttps://kcl.ac.uk// https://kcl.ac.uk/ — 301 — 243 B — 93 ms — http/1.1http://www.kcl.ac.uk/www.kcl.ac.uk/ http://www.kcl.ac.uk/ — 307 — 0 B — 1 ms — http/1.1https://www.kcl.ac.uk/www.kcl.ac.uk/ https://www.kcl.ac.uk/ — 200 — 18.7 KiB — 136 ms — h2https://www.kcl.ac.uk/siteelements/2017/css/style.min.css?version=8013490?version=8013490www.kcl.ac.uk/siteelements/2017/css... https://www.kcl.ac.uk/siteelements/2017/css/style.min.css?version=8013490?version=8013490 — 200 — 74.6 KiB — 74 ms — h2https://www.kcl.ac.uk/SiteElements/2017/js/jquery-3.2.1.min.js?version=7389704&build=171169www.kcl.ac.uk/SiteElements/2017/js/... https://www.kcl.ac.uk/SiteElements/2017/js/jquery-3.2.1.min.js?version=7389704&build=171169 — 200 — 30.6 KiB — 72 ms — h2https://cdn.cookielaw.org/consent/167b1744-c18f-47c4-bd0c-e8420eacd9e1/OtAutoBlock.jscdn.cookielaw.org/consent/167b1744-... 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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 2006ms · 527KB · £5/mo
Google Tag Manager
Tag Manager
Costly
Execution 765ms
Transfer 192 KB
Unused 33%
Monthly Cost £2/mo
www.kcl.ac.uk
Other
Costly
Execution 363ms
Transfer 19 KB
Monthly Cost £1/mo
Unattributable
Other
Costly
Execution 254ms
Transfer 0 KB
Monthly Cost £1/mo
www.kcl.ac.uk
Other
Costly
Execution 233ms
Transfer 31 KB
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Costly
Execution 223ms
Transfer 170 KB
Unused 39%
Monthly Cost £1/mo
OneTrust
Consent
Essential
Execution 167ms
Transfer 116 KB
Unused 60%
Monthly Cost £0/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 765ms and costs ~£2/month
  • www.kcl.ac.uk adds 363ms and costs ~£1/month
  • Unattributable adds 254ms and costs ~£1/month
  • www.kcl.ac.uk adds 233ms and costs ~£1/month
  • Google Tag Manager adds 223ms and costs ~£1/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 765ms 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.kcl.ac.uk takes 363ms 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 254ms 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.kcl.ac.uk takes 233ms 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

Google Tag Manager takes 223ms 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

60% of OneTrust'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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