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https://universiteitleiden.nl

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
80
GRADE
B
FIX
3
REVIEW
5
PASS
7
INFO
2

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

Mobile Desktop
34
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
89
Accessibility Measures how accessible the page is for users with disabilities. Checks color contrast, ARIA labels, and semantic HTML.
96
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
92
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.

4.81 s

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

11.64 s

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

506 ms

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

0.312

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

4.81 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.

11.81 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 (1.6 MiB of 1.6 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
D
Page Weight Inventory
Action
41 resources · 1.6 MB · 199 KB savings available
FIX
41 resources · 1.6 MB · 199 KB savings available
Warning::
Page weight 578 KB over 1MB target
Got: 1.6 MB Expected: ≤ 1 MB
Info::
~199 KB of savings available
Warning::
40 third-party resources (100% of weight)
Info::
1 resources over 200KB
41 resources · 1.6 MiB · 40 third-party 577.8 KiB over 1MB target
Image 52% · 839.4 KiB
JavaScript 35% · 556.2 KiB
Font 7% · 111.4 KiB
CSS 5% · 84.2 KiB
Document 1% · 9.2 KiB
Other 0% · 1.4 KiB

~199.0 KiB of savings available

Remove unused JavaScript: 199.0 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
1.6 MB transferred, 41 requests
REVIEW
1.6 MB transferred, 41 requests
Info::
Page weighs 3.2 MB (1.6 MB transferred)
Warning::
JavaScript is 556 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 556 KB
Info::
41 HTTP requests
Info::
Estimated 0.33 g CO2 per page load
1.6 MB / 2.4 MB budget
1.6 MB transferred
41 requests
0.33 g CO2 per page load
Images 834.5 KiB 52%
JavaScript 556.2 KiB 35%
Fonts 111.4 KiB 7%
CSS 84.2 KiB 5%
HTML 7.2 KiB 0%
Other 5.0 KiB 0%
Other 1.9 KiB 0%
Other 1.3 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
Image Optimization
Action
11 images, 0 KB saveable
REVIEW
11 images, 0 KB saveable
Warning::
https://www.universiteitleiden.nl/binaries/content... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.universiteitleiden.nl/binaries/content... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.universiteitleiden.nl/binaries/content... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
11 images 835 KB
0 oversized -0 KB
9 legacy format
3 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

C
JS Execution Cost
Action
1483ms total JS execution
REVIEW
1483ms total JS execution
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-P7S...: 345ms CPU time
Warning::
https://www.googletagmanager.com/gtag/js?id=G-0RSX...: 314ms CPU time
Warning::
Unattributable: 293ms CPU time
Warning::
https://www.googletagmanager.com/gtag/js?id=G-CRX4...: 263ms CPU time
Info::
https://www.universiteitleiden.nl/: 205ms CPU time
Warning::
Third-party scripts: 1483ms (100% of total)

Main Thread Breakdown

Script Evaluation 55% Other 19% Style & Layout 10% Script Parsing & Compilation 10% Parse HTML & CSS 3%
ScriptTotalScriptingParse/CompileParty
https://www.googletagmanager.com/gtm.js?id=GTM-P7SF446345ms291ms49ms3rd
https://www.googletagmanager.com/gtag/js?id=G-0RSX4L0ZPZ&cx=c&gtm=4e64k0314ms266ms40ms3rd
Unattributable293ms9ms0ms3rd
https://www.googletagmanager.com/gtag/js?id=G-CRX4PM593G&cx=c&gtm=4e64k0263ms220ms33ms3rd
https://www.universiteitleiden.nl/205ms8ms3ms3rd
chrome-error://chromewebdata/62ms18ms27ms3rd
C
JS Bundles
Action
16 scripts, 199 KB unused
REVIEW
16 scripts, 199 KB unused
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-P7S...: 70 KB unused (40%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=G-0RSX...: 65 KB unused (38%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=G-CRX4...: 64 KB unused (41%)
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://www.googletagmanager.com/gtm.js?id=GTM-P7SF446176 KB70 KB (40%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-0RSX4L0ZPZ&cx=c&gtm=4e64k0173 KB65 KB (38%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-CRX4PM593G&cx=c&gtm=4e64k0157 KB64 KB (41%)✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/box-more.js?v=1.223.001 KB—✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/main.js2 KB—✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/lib/jquery.js?v=1.223.0032 KB—✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/ajax.js?v=1.223.002 KB—✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/bgset.js?v=1.223.001 KB—✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/ul2.js?v=1.223.001 KB—✓3rd
https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/mobile-fit-menu-items.js?v=1.223.003 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
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
3 fonts (111 KB)
PASS
3 fonts (111 KB)
Info::
3 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

3

111 KB total

Render-blocking

0

of 3

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 FCP 4.8 s LCP 11.6 shttps://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-regular.woff2vestulapro-regular.woff2 https://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-regular.woff2 — 42.4 KiB — 42 ms — font-display: swaphttps://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-semibold.woff2vestulapro-semibold.woff2 https://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-semibold.woff2 — 42.8 KiB — 75 ms — font-display: swaphttps://www.universiteitleiden.nl/design-1.0/assets/fonts/fontello/fontello.woff2fontello.woff2 https://www.universiteitleiden.nl/design-1.0/assets/fonts/fontello/fontello.woff2 — 26.3 KiB — 109 ms — font-display: swap
vestulapro-regular.woff2 woff2 swap
Size 42 KB
Load time 42 ms
Start 388 ms
Risk FOUT — text flashes from fallback to web font
vestulapro-semibold.woff2 woff2 swap
Size 43 KB
Load time 75 ms
Start 388 ms
Risk FOUT — text flashes from fallback to web font
fontello.woff2 woff2 swap
Size 26 KB
Load time 109 ms
Start 388 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
41 requests over 1094ms
INFO
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s FCP 4.8 s LCP 11.6 shttps://universiteitleiden.nl// https://universiteitleiden.nl/ — 302 — 153 B — 116 ms — http/1.1https://www.universiteitleiden.nl/www.universiteitleiden.nl/ https://www.universiteitleiden.nl/ — 200 — 6.9 KiB — 116 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/css/ul2external/screen.css?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/css/ul2external/screen.css?v=1.223.00 — 200 — 84.2 KiB — 100 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/assets/images/zegel.pngwww.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/assets/images/zegel.png — 200 — 6.6 KiB — 114 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2/headers/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/d1400xvarwww.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2/headers/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/d1400xvar — 200 — 591.4 KiB — 426 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2/main-images/law/publiekrecht/europees-recht/nery-zarate-unsplash.jpg/nery-zarate-unsplash.jpg/d50x50www.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2/main-images/law/publiekrecht/europees-recht/nery-zarate-unsplash.jpg/nery-zarate-unsplash.jpg/d50x50 — 200 — 4.4 KiB — 38 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2student/governance-and-global-affairs/risk-talk-phd.png/risk-talk-phd.png/d50x50www.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2student/governance-and-global-affairs/risk-talk-phd.png/risk-talk-phd.png/d50x50 — 200 — 7.0 KiB — 43 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2/banners/banner-covid-boek-boin-wereldwijd-corona.jpg/banner-covid-boek-boin-wereldwijd-corona.jpg/d50x50www.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2/banners/banner-covid-boek-boin-wereldwijd-corona.jpg/banner-covid-boek-boin-wereldwijd-corona.jpg/d50x50 — 200 — 4.8 KiB — 118 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2.js?v=1.223.00 — 200 — 1.3 KiB — 117 ms — http/1.1https://www.googletagmanager.com/gtm.js?id=GTM-P7SF446www.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-P7SF446 — 200 — 176.5 KiB — 170 ms — h2https://www.universiteitleiden.nl/design-1.0/assets/images/world-map.pngwww.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/assets/images/world-map.png — 200 — 29.3 KiB — 113 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-regular.woff2www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-regular.woff2 — 200 — 42.4 KiB — 42 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-semibold.woff2www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/assets/fonts/vestulapro/vestulapro-semibold.woff2 — 200 — 42.8 KiB — 75 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/assets/fonts/fontello/fontello.woff2www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/assets/fonts/fontello/fontello.woff2 — 200 — 26.3 KiB — 109 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/lib/jquery.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/lib/jquery.js?v=1.223.00 — 200 — 32.0 KiB — 38 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/main.jswww.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/main.js — 200 — 1.8 KiB — 35 ms — http/1.1https://www.googletagmanager.com/gtag/js?id=G-0RSX4L0ZPZ&cx=c&gtm=4e64k0www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=G-0RSX4L0ZPZ&cx=c&gtm=4e64k0 — 200 — 173.0 KiB — 50 ms — h2https://www.googletagmanager.com/gtag/js?id=G-CRX4PM593G&cx=c&gtm=4e64k0www.googletagmanager.com/gtag/js?id... https://www.googletagmanager.com/gtag/js?id=G-CRX4PM593G&cx=c&gtm=4e64k0 — 200 — 156.9 KiB — 70 ms — h2https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/ajax.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/ajax.js?v=1.223.00 — 200 — 1.6 KiB — 41 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/bgset.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/bgset.js?v=1.223.00 — 200 — 1.3 KiB — 41 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/box-more.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/box-more.js?v=1.223.00 — 200 — 902 B — 41 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/mobile-fit-menu-items.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/mobile-fit-menu-items.js?v=1.223.00 — 200 — 2.6 KiB — 41 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/nav-hamburger.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/nav-hamburger.js?v=1.223.00 — 200 — 1.2 KiB — 40 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/nav-disclosure.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/nav-disclosure.js?v=1.223.00 — 200 — 1.6 KiB — 42 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/search.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/search.js?v=1.223.00 — 200 — 1.5 KiB — 40 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/suggest.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/suggest.js?v=1.223.00 — 200 — 2.0 KiB — 40 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/track-event.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/track-event.js?v=1.223.00 — 200 — 1.0 KiB — 41 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/trigger-click.js?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/scripts/ul2common/trigger-click.js?v=1.223.00 — 200 — 1016 B — 40 ms — http/1.1https://www.universiteitleiden.nl/async/spotlightcarousel?uuid=41c6a7b1-d88f-4337-ada5-f30725e6eccbwww.universiteitleiden.nl/async/spo... https://www.universiteitleiden.nl/async/spotlightcarousel?uuid=41c6a7b1-d88f-4337-ada5-f30725e6eccb — 200 — 973 B — 48 ms — http/1.1https://www.universiteitleiden.nl/async/spotlightcarousel?uuid=fc5cde43-ac54-4064-8dfe-d3a7f11c9ad0www.universiteitleiden.nl/async/spo... https://www.universiteitleiden.nl/async/spotlightcarousel?uuid=fc5cde43-ac54-4064-8dfe-d3a7f11c9ad0 — 200 — 1007 B — 77 ms — http/1.1
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s FCP 4.8 s LCP 11.6 shttps://universiteitleiden.nl// https://universiteitleiden.nl/ — 302 — 153 B — 116 ms — http/1.1https://www.universiteitleiden.nl/www.universiteitleiden.nl/ https://www.universiteitleiden.nl/ — 200 — 6.9 KiB — 116 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/css/ul2external/screen.css?v=1.223.00www.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/css/ul2external/screen.css?v=1.223.00 — 200 — 84.2 KiB — 100 ms — http/1.1https://www.universiteitleiden.nl/design-1.0/assets/images/zegel.pngwww.universiteitleiden.nl/design-1.... https://www.universiteitleiden.nl/design-1.0/assets/images/zegel.png — 200 — 6.6 KiB — 114 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2/headers/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/d1400xvarwww.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2/headers/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/260422-header-frontiers-planet-prize-jakub-zerdzicki-unsplash-.jpg/d1400xvar — 200 — 591.4 KiB — 426 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2/main-images/law/publiekrecht/europees-recht/nery-zarate-unsplash.jpg/nery-zarate-unsplash.jpg/d50x50www.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2/main-images/law/publiekrecht/europees-recht/nery-zarate-unsplash.jpg/nery-zarate-unsplash.jpg/d50x50 — 200 — 4.4 KiB — 38 ms — http/1.1https://www.universiteitleiden.nl/binaries/content/gallery/ul2student/governance-and-global-affairs/risk-talk-phd.png/risk-talk-phd.png/d50x50www.universiteitleiden.nl/binaries/... https://www.universiteitleiden.nl/binaries/content/gallery/ul2student/governance-and-global-affairs/risk-talk-phd.png/risk-talk-phd.png/d50x50 — 200 — 7.0 KiB — 43 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 1483ms · 513KB · €16/mo
Google Tag Manager
Tag Manager
Costly
Execution 345ms
Transfer 176 KB
Unused 40%
Monthly Cost €4/mo
Google Tag Manager
Tag Manager
Costly
Execution 314ms
Transfer 173 KB
Unused 38%
Monthly Cost €3/mo
Unattributable
Other
Costly
Execution 293ms
Transfer 0 KB
Monthly Cost €3/mo
Google Tag Manager
Tag Manager
Costly
Execution 263ms
Transfer 157 KB
Unused 41%
Monthly Cost €3/mo
www.universiteitleiden.nl
Other
Costly
Execution 205ms
Transfer 7 KB
Monthly Cost €2/mo
chromewebdata
Other
Optional
Execution 62ms
Transfer 0 KB
Monthly Cost €1/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 345ms and costs ~€4/month
  • Google Tag Manager adds 314ms and costs ~€3/month
  • Unattributable adds 293ms and costs ~€3/month
  • Google Tag Manager adds 263ms and costs ~€3/month
  • www.universiteitleiden.nl adds 205ms 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 345ms 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 314ms 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 293ms 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 263ms 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.universiteitleiden.nl takes 205ms 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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