Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.

SCORE

GRADE

FIX

REVIEW

PASS

INFO

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

Mobile Desktop

Performance

Accessibility

100

Best Practices

100

SEO

First Contentful Paint First Contentful Paint — how long until the browser renders the first piece of content. Under 1.8s is good.

3.84 s

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

4.65 s

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

8 ms

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

0.008

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

4.99 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.

4.70 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

9 PASS 3 REVIEW 3 FIX

Third-Party Impact

Action

100% third-party, 0 ms blocking

FIX

100% third-party, 0 ms blocking

Third-party code accounts for 100% of page weight (774.2 KiB of 774.5 KiB)

Third-party blocking time is low (0 ms)

100%

First-party Third-party

JS Execution Cost

Action

2030ms total JS execution

FIX

2030ms total JS execution

https://www.ucl.ac.uk/: 999ms CPU time

Unattributable: 643ms CPU time

https://cookie-consent.ucl.ac.uk/dist/ucl-consent....: 118ms CPU time

https://www.ucl.ac.uk/home/core/misc/drupal.init.j...: 113ms CPU time

https://cookie-consent.ucl.ac.uk/dist/ucl-consent....: 87ms CPU time

Third-party scripts: 2030ms (100% of total)

Main Thread Breakdown

Other 39% Style & Layout 32% Script Evaluation 12% Parse HTML & CSS 11% Script Parsing & Compilation 3%

Script	Total	Scripting	Parse/Compile	Party
https://www.ucl.ac.uk/	999ms	27ms	2ms	3rd
Unattributable	643ms	13ms	0ms	3rd
https://cookie-consent.ucl.ac.uk/dist/ucl-consent.blocker.min.js	118ms	90ms	8ms	3rd
https://www.ucl.ac.uk/home/core/misc/drupal.init.js?v=10.4.8	113ms	18ms	1ms	3rd
https://cookie-consent.ucl.ac.uk/dist/ucl-consent.ui-full.min.js	87ms	31ms	10ms	3rd
https://www.ucl.ac.uk/home/core/assets/vendor/jquery/jquery.min.js?v=3.7.1	69ms	50ms	14ms	3rd

HTTP Caching

Action

No cache headers

FIX

No cache headers

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.

Image Optimization

10 images, 0 KB saveable

REVIEW

10 images, 0 KB saveable

All images are well-optimized

10 images 440 KB

0 oversized -0 KB

4 legacy format

0 missing dimensions CLS risk

JS Bundles

31 scripts, 25 KB unused

REVIEW

31 scripts, 25 KB unused

https://www.ucl.ac.uk/home/core/assets/vendor/jque...: 25 KB unused (76%)

Consider code splitting or tree shaking to reduce unused code.

https://www.ucl.ac.uk/home/core/misc/ajax.js?v=10....: unminified — 14 KB could be saved

https://www.ucl.ac.uk/home/core/misc/tabbingmanage...: unminified — 3 KB could be saved

Total unused JavaScript: 25 KB

25 KB unused JavaScript

Downloaded but never executed on this page.

Script	Size	Unused	Minified	Party
https://www.ucl.ac.uk/home/core/assets/vendor/jquery/jquery.min.js?v=3.7.1	34 KB	25 KB (76%)	✓	3rd
https://www.ucl.ac.uk/home/core/misc/ajax.js?v=10.4.8	19 KB	—	✗	3rd
https://www.ucl.ac.uk/home/core/misc/tabbingmanager.js?v=10.4.8	4 KB	—	✗	3rd
https://www.ucl.ac.uk/home/core/assets/vendor/once/once.min.js?v=1.0.1	1 KB	—	✓	3rd
https://www.ucl.ac.uk/home/core/misc/drupalSettingsLoader.js?v=10.4.8	0 KB	—	✓	3rd
https://www.ucl.ac.uk/home/core/misc/drupal.js?v=10.4.8	7 KB	—	✗	3rd
https://www.ucl.ac.uk/home/core/misc/drupal.init.js?v=10.4.8	1 KB	—	✓	3rd
https://www.ucl.ac.uk/home/core/assets/vendor/tabbable/index.umd.min.js?v=6.2.0	3 KB	—	✓	3rd
https://www.ucl.ac.uk/home/modules/contrib/google_tag/js/gtag.js?tctc50	1 KB	—	✓	3rd
https://www.ucl.ac.uk/home/core/misc/progress.js?v=10.4.8	2 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

Green Hosting

Action

Whether the site is served from green-energy infrastructure

REVIEW

Green Hosting

No green hosting detected

Page Weight Budget

775 KB transferred, 87 requests

PASS

775 KB transferred, 87 requests

Page weighs 2.0 MB (775 KB transferred)

87 HTTP requests

Estimated 0.16 g CO2 per page load

775 KB transferred

87 requests

0.16 g CO2 per page load

Images 440.0 KiB 57%

JavaScript 115.5 KiB 15%

Fonts 90.6 KiB 12%

Other 73.3 KiB 9%

CSS 32.4 KiB 4%

HTML 21.8 KiB 3%

Other 951 B 0%

0 2.4 MB 4.9 MB

Under budget

A+

Text Compression

All text resources are compressed

PASS

All text resources are compressed

All text resources are properly compressed.

A+

Font Loading

1 fonts (91 KB)

PASS

1 fonts (91 KB)

1 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

91 KB total

Render-blocking

of 1

Dominant font-display

swap

Most common across fonts

Font loading timeline

Transfer FOIT (block) FOUT (swap)

DMSans[opsz,wght].woff2 woff2 swap

Size 91 KB

Load time 43 ms

Start 1.3 s

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

No caching issues found

All static resources have appropriate caching headers.

A+

Critical Rendering Path

No render-blocking resources

PASS

No render-blocking resources

No render-blocking resources detected

A+

Resource Hints

No optimization needed

PASS

No optimization needed

No resource hint issues

Page Weight Inventory

87 resources · 775 KB · 25 KB savings available

PASS

87 resources · 775 KB · 25 KB savings available

Within 1MB page-weight target

Got: 775 KB

86 third-party resources (100% of weight)

87 resources · 774.5 KiB · 86 third-party Within 1MB page-weight target

Image 66% · 513.1 KiB

JavaScript 15% · 115.5 KiB

Font 12% · 90.6 KiB

CSS 4% · 32.4 KiB

Document 3% · 22.0 KiB

Other 0% · 951 B

~25.4 KiB of savings available

Remove unused JavaScript: 25.4 KiB

#	Resource	Type	Size	Cache	Unused	3rd party	Issues

87 resources			774.5 KiB

A+

Render-Blocking Resources

No render-blocking resources detected

PASS

No render-blocking resources detected

No render-blocking resources detected in <head>

A+

Third-Party Resources

No third-party resources detected

PASS

No third-party resources detected

Network Waterfall

87 requests over 1683ms

INFO

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 2030ms · 84KB · £21/mo

Script	Category	Execution	Transfer	Unused	Monthly Cost	Verdict
www.ucl.ac.uk www.ucl.ac.uk	Other	999ms	22 KB	—	£10/mo	Costly
Unattributable Unattributable	Other	643ms	0 KB	—	£7/mo	Costly
cookie-consent.ucl.ac.uk cookie-consent.ucl.ac.uk	Other	118ms	13 KB	—	£1/mo	Optional
www.ucl.ac.uk www.ucl.ac.uk	Other	113ms	1 KB	—	£1/mo	Optional
cookie-consent.ucl.ac.uk cookie-consent.ucl.ac.uk	Other	87ms	15 KB	—	£1/mo	Optional
www.ucl.ac.uk www.ucl.ac.uk	Other	69ms	34 KB	76%	£1/mo	Optional

www.ucl.ac.uk

Other

Costly

Execution 999ms

Transfer 22 KB

Monthly Cost £10/mo

Unattributable

Other

Costly

Execution 643ms

Transfer 0 KB

Monthly Cost £7/mo

cookie-consent.ucl.ac.uk

Other

Optional

Execution 118ms

Transfer 13 KB

Monthly Cost £1/mo

www.ucl.ac.uk

Other

Optional

Execution 113ms

Transfer 1 KB

Monthly Cost £1/mo

cookie-consent.ucl.ac.uk

Other

Optional

Execution 87ms

Transfer 15 KB

Monthly Cost £1/mo

www.ucl.ac.uk

Other

Optional

Execution 69ms

Transfer 34 KB

Unused 76%

Monthly Cost £1/mo

These scripts may cost more than they're worth

www.ucl.ac.uk adds 999ms and costs ~£10/month
Unattributable adds 643ms and costs ~£7/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

www.ucl.ac.uk takes 999ms 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 643ms 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

76% of www.ucl.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

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.