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Performance

· 14 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
80
GRADE
B
FIX
1
REVIEW
4
PASS
7
INFO
2

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

Mobile Desktop
71
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
96
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.
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.

2.78 s

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

3.80 s

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

492 ms

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

0.001

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

3.30 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.

10.63 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
14
7 PASS 4 REVIEW 1 FIX
D
JS Execution Cost
Action
2490ms total JS execution
FIX
2490ms total JS execution
Warning::
https://www.cam.ac.uk/: 542ms CPU time
Warning::
https://www.googletagmanager.com/gtag/js?id=G-P8Q1...: 442ms CPU time
Warning::
Unattributable: 413ms CPU time
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-NV9...: 339ms CPU time
Warning::
https://cc.cdn.civiccomputing.com/9/cookieControl-...: 283ms CPU time
Warning::
Third-party scripts: 1818ms (73% of total)

Main Thread Breakdown

Script Evaluation 49% Other 18% Style & Layout 14% Script Parsing & Compilation 11% Parse HTML & CSS 4%
ScriptTotalScriptingParse/CompileParty
https://www.cam.ac.uk/542ms39ms12ms1st
https://www.googletagmanager.com/gtag/js?id=G-P8Q1QT5W4K&cx=c&gtm=4e6411442ms368ms69ms3rd
Unattributable413ms8ms0ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-NV99KVN339ms262ms48ms3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js283ms230ms51ms3rd
https://www.googletagmanager.com/gtag/js?id=G-KV2LE1CRC2&cx=c&gtm=4e6411229ms167ms59ms3rd
https://www.cam.ac.uk/sites/all/themes/fresh/js/libs/jquery-3.5.1.min.js?v=3.5.1129ms70ms11ms1st
https://js-agent.newrelic.com/nr-rum-1.312.1.min.js113ms94ms7ms3rd
C
Page Weight Budget
Action
1.8 MB transferred, 65 requests
REVIEW
1.8 MB transferred, 65 requests
Info::
Page weighs 3.4 MB (1.8 MB transferred)
Warning::
JavaScript is 684 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 684 KB
Info::
65 HTTP requests
Info::
Estimated 0.37 g CO2 per page load
1.8 MB / 2.4 MB budget
1.8 MB transferred
65 requests
0.37 g CO2 per page load
Images 971.2 KiB 53%
JavaScript 684.5 KiB 38%
Fonts 88.3 KiB 5%
CSS 46.8 KiB 3%
HTML 22.2 KiB 1%
Other 1.7 KiB 0%
Other 1.2 KiB 0%
Other 595 B 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
33 images, 0 KB saveable
REVIEW
33 images, 0 KB saveable
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/all/themes/fresh/image... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/all/themes/fresh/image... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://www.cam.ac.uk/sites/default/files/styles/h... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
33 images 971 KB
0 oversized -0 KB
33 legacy format
16 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

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
15 scripts, 276 KB unused
REVIEW
15 scripts, 276 KB unused
Warning::
https://www.googletagmanager.com/gtag/js?id=G-P8Q1...: 69 KB unused (40%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=G-KV2L...: 65 KB unused (42%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-NV9...: 63 KB unused (48%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://cc.cdn.civiccomputing.com/9/cookieControl-...: 57 KB unused (47%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.cam.ac.uk/sites/all/themes/fresh/js/li...: 22 KB unused (83%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 276 KB
276 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.googletagmanager.com/gtag/js?id=G-P8Q1QT5W4K&cx=c&gtm=4e6411175 KB69 KB (40%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-KV2LE1CRC2&cx=c&gtm=4e6411155 KB65 KB (42%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-NV99KVN131 KB63 KB (48%)✓3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js123 KB57 KB (47%)✓3rd
https://www.cam.ac.uk/sites/all/themes/fresh/js/libs/bootstrap.bundle.min.js?tankhw26 KB22 KB (83%)✓1st
https://www.cam.ac.uk/misc/jquery-html-prefilter-3.5.0-backport.js?v=1.12.45 KB—✗1st
https://www.cam.ac.uk/profiles/cambridge/modules/contrib/jquery_update/js/jquery_browser.js?v=0.0.11 KB—✓1st
https://www.cam.ac.uk/profiles/cambridge/modules/contrib/entityreference/js/entityreference.js?tankhw1 KB—✓1st
https://www.cam.ac.uk/sites/all/themes/fresh/js/custom.js?tankhw0 KB—✓1st
https://www.cam.ac.uk/misc/drupal.js?tankhw8 KB—✗1st

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

C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

A
Third-Party Impact
38% third-party, 0 ms blocking
PASS
38% third-party, 0 ms blocking
Info::
Third-party code accounts for 38% of page weight (695.4 KiB of 1.8 MiB)
Info::
Third-party blocking time is low (0 ms)
62%
38%
First-party Third-party
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 (88 KB)
PASS
4 fonts (88 KB)
Info::
4 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

4

88 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 FCP 2.8 s LCP 3.8 shttps://fonts.gstatic.com/s/lato/v25/S6uyw4BMUTPHjx4wXiWtFCc.woff2S6uyw4BMUTPHjx4wXiWtFCc... https://fonts.gstatic.com/s/lato/v25/S6uyw4BMUTPHjx4wXiWtFCc.woff2 — 13.7 KiB — 0 ms — font-display: swaphttps://fonts.gstatic.com/s/lato/v25/S6u9w4BMUTPHh6UVSwiPGQ3q5d0.woff2S6u9w4BMUTPHh6UVSwiPGQ3... https://fonts.gstatic.com/s/lato/v25/S6u9w4BMUTPHh6UVSwiPGQ3q5d0.woff2 — 13.9 KiB — 0 ms — font-display: swaphttps://fonts.gstatic.com/s/lato/v25/S6u9w4BMUTPHh7USSwiPGQ3q5d0.woff2S6u9w4BMUTPHh7USSwiPGQ3... https://fonts.gstatic.com/s/lato/v25/S6u9w4BMUTPHh7USSwiPGQ3q5d0.woff2 — 13.6 KiB — 0 ms — font-display: swaphttps://fonts.gstatic.com/s/opensans/v40/memvYaGs126MiZpBA-UvWbX2vVnXBbObj2OVTS-muw.woff2memvYaGs126MiZpBA-UvWbX... https://fonts.gstatic.com/s/opensans/v40/memvYaGs126MiZpBA-UvWbX2vVnXBbObj2OVTS-muw.woff2 — 47.1 KiB — 0 ms — font-display: swap
S6uyw4BMUTPHjx4wXiWtFCc... woff2 swap
Size 14 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
S6u9w4BMUTPHh6UVSwiPGQ3... woff2 swap
Size 14 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
S6u9w4BMUTPHh7USSwiPGQ3... woff2 swap
Size 14 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
memvYaGs126MiZpBA-UvWbX... woff2 swap
Size 47 KB
Load time 0 ms
Start 0 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+
HTTP Caching
max-age=900 (15 minutes)
PASS
max-age=900 (15 minutes)
Info::
Cache-Control header is set
Got: public, max-age=900
Info::
Effective TTL: 15 minutes (max-age=900)
Info::
ETag present — enables conditional (304) responses
Info::
Last-Modified header present
Cache-Control

public, max-age=900

DirectiveValueMeaning
public—Any cache may store this response
max-age900Cache for 15 minutes
Effective TTL: 900 seconds
Network Waterfall
65 requests over 0ms
INFO

Performance analysis not available for this result.

Third-Party Script Cost
Per-script blocking time, transfer cost, and cache headers
INFO
73% of JavaScript execution is third-party
First-party Third-party 1818ms · 604KB · £19/mo
Google Tag Manager
Tag Manager
Costly
Execution 442ms
Transfer 175 KB
Unused 40%
Monthly Cost £5/mo
Unattributable
Other
Costly
Execution 413ms
Transfer 0 KB
Monthly Cost £4/mo
Google Tag Manager
Tag Manager
Costly
Execution 339ms
Transfer 131 KB
Unused 48%
Monthly Cost £4/mo
cc.cdn.civiccomputing.com
Other
Costly
Execution 283ms
Transfer 123 KB
Unused 47%
Monthly Cost £3/mo
Google Tag Manager
Tag Manager
Costly
Execution 229ms
Transfer 155 KB
Unused 42%
Monthly Cost £2/mo
js-agent.newrelic.com
Other
Optional
Execution 113ms
Transfer 18 KB
Monthly Cost £1/mo

These scripts may cost more than they're worth

  • Google Tag Manager adds 442ms and costs ~£5/month
  • Unattributable adds 413ms and costs ~£4/month
  • Google Tag Manager adds 339ms and costs ~£4/month
  • cc.cdn.civiccomputing.com adds 283ms and costs ~£3/month
  • Google Tag Manager adds 229ms and costs ~£2/month

73% 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 442ms 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 413ms 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 339ms 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

cc.cdn.civiccomputing.com takes 283ms 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 229ms 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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