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Performance

· 14 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
80
GRADE
B
FIX
0
REVIEW
5
PASS
7
INFO
2

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

Mobile Desktop
99
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.

737 ms

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

872 ms

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

24 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.

953 ms

Time to Interactive Time to Interactive — how long until the page is fully interactive and responds to user input. Under 3.8s is good.

1.91 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 5 REVIEW
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 686 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 686 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 971.6 KiB 53%
JavaScript 686.3 KiB 38%
Fonts 88.3 KiB 5%
CSS 46.9 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/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/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 972 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

B
JS Execution Cost
567ms total JS execution
REVIEW
567ms total JS execution
Info::
https://www.cam.ac.uk/: 138ms CPU time
Info::
Unattributable: 104ms CPU time
Info::
https://www.googletagmanager.com/gtag/js?id=G-KV2L...: 97ms CPU time
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-NV9...: 86ms CPU time
Info::
https://www.googletagmanager.com/gtag/js?id=G-P8Q1...: 72ms CPU time
Warning::
Third-party scripts: 430ms (76% of total)

Main Thread Breakdown

Script Evaluation 49% Other 21% Style & Layout 12% Script Parsing & Compilation 11% Parse HTML & CSS 4%
ScriptTotalScriptingParse/CompileParty
https://www.cam.ac.uk/138ms10ms3ms1st
Unattributable104ms2ms0ms3rd
https://www.googletagmanager.com/gtag/js?id=G-KV2LE1CRC2&cx=c&gtm=4e6420h297ms82ms15ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-NV99KVN86ms66ms12ms3rd
https://www.googletagmanager.com/gtag/js?id=G-P8Q1QT5W4K&cx=c&gtm=4e6420h272ms54ms17ms3rd
https://cc.cdn.civiccomputing.com/9/cookieControl-9.x.min.js70ms55ms14ms3rd
C
JS Bundles
Action
15 scripts, 277 KB unused
REVIEW
15 scripts, 277 KB unused
Warning::
https://www.googletagmanager.com/gtag/js?id=G-P8Q1...: 68 KB unused (39%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=G-KV2L...: 66 KB unused (43%)
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: 277 KB
277 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.googletagmanager.com/gtag/js?id=G-P8Q1QT5W4K&cx=c&gtm=4e6420h2176 KB68 KB (39%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-KV2LE1CRC2&cx=c&gtm=4e6420h2156 KB66 KB (43%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-NV99KVN132 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 (697.1 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 500 ms 1.0 s FCP 737 ms LCP 872 mshttps://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
76% of JavaScript execution is third-party
First-party Third-party 430ms · 587KB · £4/mo
Unattributable
Other
Optional
Execution 104ms
Transfer 0 KB
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 97ms
Transfer 156 KB
Unused 43%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 86ms
Transfer 132 KB
Unused 48%
Monthly Cost £1/mo
Google Tag Manager
Tag Manager
Optional
Execution 72ms
Transfer 176 KB
Unused 39%
Monthly Cost £1/mo
cc.cdn.civiccomputing.com
Other
Optional
Execution 70ms
Transfer 123 KB
Unused 47%
Monthly Cost £1/mo

76% 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

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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