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https://sheffield.ac.uk

Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
83
GRADE
B
FIX
1
REVIEW
6
PASS
8
INFO
2

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

Mobile Desktop
59
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
99
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.

4.18 s

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

11.13 s

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

95 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.

7.62 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.

11.31 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
8 PASS 6 REVIEW 1 FIX
D
Page Weight Inventory
Action
51 resources · 1.9 MB · 141 KB savings available
FIX
51 resources · 1.9 MB · 141 KB savings available
Warning::
Page weight 947 KB over 1MB target
Got: 1.9 MB Expected: ≤ 1 MB
Info::
~141 KB of savings available
Warning::
30 third-party resources (47% of weight)
Info::
2 resources over 200KB
51 resources · 1.9 MiB · 30 third-party 947.0 KiB over 1MB target
Image 60% · 1.2 MiB
Font 18% · 362.0 KiB
JavaScript 16% · 315.7 KiB
CSS 3% · 64.4 KiB
Document 1% · 24.4 KiB
Other 1% · 16.5 KiB

~141.2 KiB of savings available

Remove unused JavaScript: 141.2 KiB
C
Page Weight Budget
Action
1.9 MB transferred, 51 requests
REVIEW
1.9 MB transferred, 51 requests
Info::
Page weighs 3.3 MB (1.9 MB transferred)
Warning::
Images are 1.1 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 1.1 MB
Info::
Fonts are 362 KB — consider subsetting or using system fonts
Subset fonts to include only used characters, or switch to system font stacks.
Got: 362 KB
Info::
51 HTTP requests
Info::
Estimated 0.40 g CO2 per page load
1.9 MB / 2.4 MB budget
1.9 MB transferred
51 requests
0.40 g CO2 per page load
Images 1.1 MiB 59%
Fonts 362.0 KiB 18%
JavaScript 290.2 KiB 15%
CSS 59.8 KiB 3%
Other 44.5 KiB 2%
HTML 24.4 KiB 1%
Other 22.4 KiB 1%
Other 2.6 KiB 0%
0 2.4 MB 4.9 MB
Approaching limit

Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.

Why this matters

Image bandwidth is high — modern formats (WebP/AVIF) and resizing typically cut it 50%+.

Source: web.dev

Subset fonts to include only used characters, or switch to system font stacks.

Why this matters

Font payload is large — subset to used glyphs only, or fall back to system fonts (zero load time).

Source: web.dev font loading

B
Third-Party Impact
47% third-party, 0 ms blocking
REVIEW
47% third-party, 0 ms blocking
Info::
Third-party code accounts for 47% of page weight (923.8 KiB of 1.9 MiB)
Info::
Third-party blocking time is low (0 ms)
53%
47%
First-party Third-party
B
Image Optimization
27 images, 0 KB saveable
REVIEW
27 images, 0 KB saveable
Warning::
https://sheffield.ac.uk/themes/custom/uos_public/i... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
27 images 1.1 MB
0 oversized -0 KB
26 legacy format
1 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

C
JS Execution Cost
Action
1333ms total JS execution
REVIEW
1333ms total JS execution
Warning::
https://cdn.sheffield.ac.uk/sites/default/files/js...: 631ms CPU time
Warning::
Unattributable: 389ms CPU time
Info::
https://cdn-ukwest.onetrust.com/scripttemplates/20...: 223ms CPU time
Info::
https://sheffield.ac.uk/: 90ms CPU time
Warning::
Third-party scripts: 1243ms (93% of total)

Main Thread Breakdown

Script Evaluation 38% Other 26% Style & Layout 22% Script Parsing & Compilation 7% Parse HTML & CSS 6%
ScriptTotalScriptingParse/CompileParty
https://cdn.sheffield.ac.uk/sites/default/files/js/js_jp6Lz0rnhBXaGt8cRu-KthubP_qJo3NPhvqQHjM6Kpo.js?scope=footer&delta=0&language=en&theme=uos_public&include=eJxNjEEKgDAMBD9U7JNCm0ap1q7G5KCvFxHE28wchhcp1aCUmKGlosePhlHRTXoJ8-6iJ3ml5AbGujUxiX8JjoM2z61ynBpyauFyzTTCe0n2jN98A0vELbc631ms260ms43ms3rd
Unattributable389ms52ms0ms3rd
https://cdn-ukwest.onetrust.com/scripttemplates/202510.2.0/otBannerSdk.js223ms151ms43ms3rd
https://sheffield.ac.uk/90ms6ms2ms1st
B
JS Bundles
7 scripts, 141 KB unused
REVIEW
7 scripts, 141 KB unused
Warning::
https://cdn.sheffield.ac.uk/sites/default/files/js...: 74 KB unused (53%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://cdn-ukwest.onetrust.com/scripttemplates/20...: 67 KB unused (54%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 141 KB
141 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://cdn.sheffield.ac.uk/sites/default/files/js/js_jp6Lz0rnhBXaGt8cRu-KthubP_qJo3NPhvqQHjM6Kpo.js?scope=footer&delta=0&language=en&theme=uos_public&include=eJxNjEEKgDAMBD9U7JNCm0ap1q7G5KCvFxHE28wchhcp1aCUmKGlosePhlHRTXoJ8-6iJ3ml5AbGujUxiX8JjoM2z61ynBpyauFyzTTCe0n2jN98A0vELbc140 KB74 KB (53%)✓3rd
https://cdn-ukwest.onetrust.com/scripttemplates/202510.2.0/otBannerSdk.js124 KB67 KB (54%)✓3rd
https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/OtAutoBlock.js17 KB—✓3rd
https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.js9 KB—✓3rd
https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/a3110129-5f04-4e81-9c5e-b494a7836315.json2 KB—✓3rd
https://cdn.sheffield.ac.uk/sites/default/files/google_tag/primary/google_tag.script.js?tdkxqj1 KB—✓3rd
https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/164daec9-03f9-4ee5-a84f-63820efdac36/en.json23 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

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
6 fonts (362 KB)
PASS
6 fonts (362 KB)
Info::
6 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

6

362 KB total

Render-blocking

0

of 6

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 FCP 4.2 s LCP 11.1 shttps://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-sans-3-v15-latin-regular.woff2source-sans-3-v15-latin... https://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-sans-3-v15-latin-regular.woff2 — 15.7 KiB — 6 ms — font-display: swaphttps://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-sans-3-v15-latin-700.woff2source-sans-3-v15-latin... https://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-sans-3-v15-latin-700.woff2 — 15.6 KiB — 10 ms — font-display: swaphttps://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-serif-4-v8-latin-700.woff2source-serif-4-v8-latin... https://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-serif-4-v8-latin-700.woff2 — 21.4 KiB — 6 ms — font-display: swaphttps://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-sans-3-v15-latin-900.woff2source-sans-3-v15-latin... https://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/source-sans-3-v15-latin-900.woff2 — 14.6 KiB — 6 ms — font-display: swaphttps://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/HotMess.ttfHotMess.ttf https://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/HotMess.ttf — 214.7 KiB — 108 ms — font-display: swaphttps://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/SourceSerif4Roman-Regular.woff2SourceSerif4Roman-Regul... https://cdn.sheffield.ac.uk/themes/custom/common/uos/fonts/SourceSerif4Roman-Regular.woff2 — 80.2 KiB — 9 ms — font-display: swap
source-sans-3-v15-latin... woff2 swap
Size 16 KB
Load time 6 ms
Start 629 ms
Risk FOUT — text flashes from fallback to web font
source-sans-3-v15-latin... woff2 swap
Size 16 KB
Load time 10 ms
Start 629 ms
Risk FOUT — text flashes from fallback to web font
source-serif-4-v8-latin... woff2 swap
Size 21 KB
Load time 6 ms
Start 629 ms
Risk FOUT — text flashes from fallback to web font
source-sans-3-v15-latin... woff2 swap
Size 15 KB
Load time 6 ms
Start 630 ms
Risk FOUT — text flashes from fallback to web font
HotMess.ttf ttf swap
Size 215 KB
Load time 108 ms
Start 630 ms
Risk FOUT — text flashes from fallback to web font
  • Convert to woff2 for ~30% smaller file size
  • Subset this font — over 100 KB suggests Latin Extended or full glyph coverage that most pages don't need
SourceSerif4Roman-Regul... woff2 swap
Size 80 KB
Load time 9 ms
Start 631 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)

Fixing the unchecked items could save ~172 KB and ~344 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
A+
HTTP Caching
max-age=900 (15 minutes)
PASS
max-age=900 (15 minutes)
Info::
Cache-Control header is set
Got: max-age=900, public, s-maxage=604800
Info::
Effective TTL: 15 minutes (max-age=900)
Info::
CDN TTL: 1 week (s-maxage=604800)
Info::
ETag present — enables conditional (304) responses
Info::
Last-Modified header present
Cache-Control

max-age=900, public, s-maxage=604800

DirectiveValueMeaning
max-age900Cache for 15 minutes
public—Any cache may store this response
s-maxage604800CDN caches for 1 week
Effective TTL: 900 seconds
Network Waterfall
51 requests over 905ms
INFO
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s FCP 4.2 s LCP 11.1 shttps://sheffield.ac.uk// https://sheffield.ac.uk/ — 200 — 24.4 KiB — 166 ms — http/1.1https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/OtAutoBlock.jscdn-ukwest.onetrust.com/consent/a31... https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/OtAutoBlock.js — 200 — 16.8 KiB — 122 ms — h2https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.jscdn-ukwest.onetrust.com/scripttempl... https://cdn-ukwest.onetrust.com/scripttemplates/otSDKStub.js — 200 — 9.1 KiB — 110 ms — h2https://cdn.sheffield.ac.uk/sites/default/files/css/css_LEqc-iG5QeVPDkBugYzgSyJGoZbo1OGkJByYYLXq8yE.css?delta=0&language=en&theme=uos_public&include=eJxNj2tuwzAMgy_kxkcSZFtptTmWq0eG7PRLg2HdH4H6SAlg_aTGLgpYq2hjGflPLavKcBot0XD2A2gr1HLF6aebPp5BekAwYLhU2WYnp_x_SR0PCYfGVmU_01kGVelpouJdcT4sN42JfXmTJcaM0tke1JKSTRnGO4Fj6QQrdyfNb367eLLDnLZc0CiFGFwvar53KdjTzvRl-ZrLJi3Og-_QAqvEaPiq85v8AQBhbFscdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/css/css_LEqc-iG5QeVPDkBugYzgSyJGoZbo1OGkJByYYLXq8yE.css?delta=0&language=en&theme=uos_public&include=eJxNj2tuwzAMgy_kxkcSZFtptTmWq0eG7PRLg2HdH4H6SAlg_aTGLgpYq2hjGflPLavKcBot0XD2A2gr1HLF6aebPp5BekAwYLhU2WYnp_x_SR0PCYfGVmU_01kGVelpouJdcT4sN42JfXmTJcaM0tke1JKSTRnGO4Fj6QQrdyfNb367eLLDnLZc0CiFGFwvar53KdjTzvRl-ZrLJi3Og-_QAqvEaPiq85v8AQBhbFs — 200 — 4.8 KiB — 40 ms — h2https://cdn.sheffield.ac.uk/sites/default/files/css/css_w--SeWAlIkBVWx_reufqqGwWA9X_xBWUqR8c39mhGdI.css?delta=1&language=en&theme=uos_public&include=eJxNj2tuwzAMgy_kxkcSZFtptTmWq0eG7PRLg2HdH4H6SAlg_aTGLgpYq2hjGflPLavKcBot0XD2A2gr1HLF6aebPp5BekAwYLhU2WYnp_x_SR0PCYfGVmU_01kGVelpouJdcT4sN42JfXmTJcaM0tke1JKSTRnGO4Fj6QQrdyfNb367eLLDnLZc0CiFGFwvar53KdjTzvRl-ZrLJi3Og-_QAqvEaPiq85v8AQBhbFscdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/css/css_w--SeWAlIkBVWx_reufqqGwWA9X_xBWUqR8c39mhGdI.css?delta=1&language=en&theme=uos_public&include=eJxNj2tuwzAMgy_kxkcSZFtptTmWq0eG7PRLg2HdH4H6SAlg_aTGLgpYq2hjGflPLavKcBot0XD2A2gr1HLF6aebPp5BekAwYLhU2WYnp_x_SR0PCYfGVmU_01kGVelpouJdcT4sN42JfXmTJcaM0tke1JKSTRnGO4Fj6QQrdyfNb367eLLDnLZc0CiFGFwvar53KdjTzvRl-ZrLJi3Og-_QAqvEaPiq85v8AQBhbFs — 200 — 55.0 KiB — 40 ms — h2https://sheffield.ac.uk/themes/custom/uos_public/images/logos/uos-crest.svg/themes/custom/uos_public/images/lo... https://sheffield.ac.uk/themes/custom/uos_public/images/logos/uos-crest.svg — 200 — 16.7 KiB — 46 ms — http/1.1https://sheffield.ac.uk/sites/default/files/styles/main_home/public/2026-03/Cover_v03_large%20%281%29.jpg?h=76294784&itok=WSJ0MgKy/sites/default/files/styles/main_ho... https://sheffield.ac.uk/sites/default/files/styles/main_home/public/2026-03/Cover_v03_large%20%281%29.jpg?h=76294784&itok=WSJ0MgKy — 200 — 207.0 KiB — 121 ms — http/1.1https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-03/Cover_v03_large%20%281%29.jpg?h=647c1a54&itok=nTMw6NvY/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-03/Cover_v03_large%20%281%29.jpg?h=647c1a54&itok=nTMw6NvY — 200 — 56.9 KiB — 82 ms — http/1.1https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/a3110129-5f04-4e81-9c5e-b494a7836315.jsoncdn-ukwest.onetrust.com/consent/a31... https://cdn-ukwest.onetrust.com/consent/a3110129-5f04-4e81-9c5e-b494a7836315/a3110129-5f04-4e81-9c5e-b494a7836315.json — 200 — 2.4 KiB — 84 ms — h2https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2019-02/economics_undergrads.jpg?h=7eb62d8e&itok=P-TU2V11/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2019-02/economics_undergrads.jpg?h=7eb62d8e&itok=P-TU2V11 — 200 — 31.6 KiB — 44 ms — http/1.1https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2020-03/pgde-30564.jpg?h=82e026d0&itok=t5YkI-Ey/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2020-03/pgde-30564.jpg?h=82e026d0&itok=t5YkI-Ey — 200 — 27.2 KiB — 49 ms — http/1.1https://cdn.sheffield.ac.uk/sites/default/files/js/js_jp6Lz0rnhBXaGt8cRu-KthubP_qJo3NPhvqQHjM6Kpo.js?scope=footer&delta=0&language=en&theme=uos_public&include=eJxNjEEKgDAMBD9U7JNCm0ap1q7G5KCvFxHE28wchhcp1aCUmKGlosePhlHRTXoJ8-6iJ3ml5AbGujUxiX8JjoM2z61ynBpyauFyzTTCe0n2jN98A0vELbccdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/js/js_jp6Lz0rnhBXaGt8cRu-KthubP_qJo3NPhvqQHjM6Kpo.js?scope=footer&delta=0&language=en&theme=uos_public&include=eJxNjEEKgDAMBD9U7JNCm0ap1q7G5KCvFxHE28wchhcp1aCUmKGlosePhlHRTXoJ8-6iJ3ml5AbGujUxiX8JjoM2z61ynBpyauFyzTTCe0n2jN98A0vELbc — 200 — 139.7 KiB — 234 ms — h2https://cdn.sheffield.ac.uk/sites/default/files/google_tag/primary/google_tag.script.js?tdkxqjcdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/google_tag/primary/google_tag.script.js?tdkxqj — 200 — 548 B — 7 ms — h2https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2019-08/31306.jpg?h=97397f07&itok=cpFObRoZ/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2019-08/31306.jpg?h=97397f07&itok=cpFObRoZ — 200 — 34.5 KiB — 83 ms — http/1.1https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2022-09/GettyImages-932767346.jpg?h=b5e4e16b&itok=LicoIT0h/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2022-09/GettyImages-932767346.jpg?h=b5e4e16b&itok=LicoIT0h — 200 — 36.0 KiB — 94 ms — http/1.1https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2025-06/top-100.png.jpg?itok=SEBNBATYcdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2025-06/top-100.png.jpg?itok=SEBNBATY — 200 — 24.2 KiB — 177 ms — h2https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-04/Thought%20Leadership%20March%2026%20Meihong%20Wang.jpg?h=e19ee1b0&itok=msB7npRK/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-04/Thought%20Leadership%20March%2026%20Meihong%20Wang.jpg?h=e19ee1b0&itok=msB7npRK — 200 — 96.5 KiB — 131 ms — http/1.1https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2024-10/Firth%20Court%20with%20blue%20sky%2031317.jpg?h=15528680&itok=NgGBmXOU/sites/default/files/styles/mobile_... https://sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2024-10/Firth%20Court%20with%20blue%20sky%2031317.jpg?h=15528680&itok=NgGBmXOU — 200 — 82.3 KiB — 204 ms — http/1.1https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2019-09/Stromboli.JPG.jpg?itok=4UCGxwG7cdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2019-09/Stromboli.JPG.jpg?itok=4UCGxwG7 — 200 — 18.7 KiB — 8 ms — h2https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2023-01/player-thumb.png.jpg?itok=sUaKVbSdcdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2023-01/player-thumb.png.jpg?itok=sUaKVbSd — 200 — 36.4 KiB — 183 ms — h2https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-04/IMG_6084.jpeg.jpg?itok=gz9-k3sCcdn.sheffield.ac.uk/sites/default/f... https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-04/IMG_6084.jpeg.jpg?itok=gz9-k3sC — 200 — 48.1 KiB — 6 ms — h2https://cdn.sheffield.ac.uk/sites/default/files/styles/mobile_single_column_2x/public/2026-04/Student%20Hackathon%20Engineering.jpeg.jpg?itok=C-BUiz_4cdn.sheffield.ac.uk/sites/default/f... 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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
93% of JavaScript execution is third-party
First-party Third-party 1243ms · 264KB · £13/mo
cdn.sheffield.ac.uk
Other
Costly
Execution 631ms
Transfer 140 KB
Unused 53%
Monthly Cost £7/mo
Unattributable
Other
Costly
Execution 389ms
Transfer 0 KB
Monthly Cost £4/mo
cdn-ukwest.onetrust.com
Other
Costly
Execution 223ms
Transfer 124 KB
Unused 54%
Monthly Cost £2/mo

These scripts may cost more than they're worth

  • cdn.sheffield.ac.uk adds 631ms and costs ~£7/month
  • Unattributable adds 389ms and costs ~£4/month
  • cdn-ukwest.onetrust.com adds 223ms and costs ~£2/month

93% 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

cdn.sheffield.ac.uk takes 631ms 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

53% of cdn.sheffield.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

Unattributable takes 389ms 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

cdn-ukwest.onetrust.com 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

54% of cdn-ukwest.onetrust.com'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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