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Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
82
GRADE
B
FIX
2
REVIEW
6
PASS
7
INFO
2

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

Mobile Desktop
53
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
90
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.

6.14 s

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

9.60 s

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

265 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.

6.98 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.

9.67 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 6 REVIEW 2 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 (1.3 MiB of 1.3 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
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.

B
Page Weight Budget
1.3 MB transferred, 29 requests
REVIEW
1.3 MB transferred, 29 requests
Info::
Page weighs 1.9 MB (1.3 MB transferred)
Warning::
JavaScript is 503 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 503 KB
Info::
29 HTTP requests
Info::
Estimated 0.27 g CO2 per page load
1.3 MB / 2.4 MB budget
1.3 MB transferred
29 requests
0.27 g CO2 per page load
Images 581.6 KiB 45%
JavaScript 502.7 KiB 38%
Fonts 129.4 KiB 10%
CSS 76.7 KiB 6%
HTML 12.1 KiB 1%
Other 2.4 KiB 0%
Other 1.1 KiB 0%
Other 730 B 0%
0 2.4 MB 4.9 MB
Under budget

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

B
Image Optimization
7 images, 0 KB saveable
REVIEW
7 images, 0 KB saveable
Warning::
https://web.mit.edu/files/images/202604/MIT-Brian-... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
7 images 582 KB
0 oversized -0 KB
7 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
1765ms total JS execution
REVIEW
1765ms total JS execution
Warning::
https://web.mit.edu/themes/mit/assets/js/main.js?t...: 754ms CPU time
Warning::
https://web.mit.edu/: 369ms CPU time
Warning::
Unattributable: 295ms CPU time
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-M8M...: 275ms CPU time
Info::
https://web.mit.edu/themes/mit/assets/css/main.css...: 72ms CPU time
Warning::
Third-party scripts: 1765ms (100% of total)

Main Thread Breakdown

Script Evaluation 40% Other 31% Style & Layout 16% Script Parsing & Compilation 7% Parse HTML & CSS 5%
ScriptTotalScriptingParse/CompileParty
https://web.mit.edu/themes/mit/assets/js/main.js?t5zcu8754ms431ms62ms3rd
https://web.mit.edu/369ms20ms3ms3rd
Unattributable295ms10ms0ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-M8MGV4P275ms200ms56ms3rd
https://web.mit.edu/themes/mit/assets/css/main.css?t5zcu872ms0ms0ms3rd
C
JS Bundles
Action
6 scripts, 252 KB unused
REVIEW
6 scripts, 252 KB unused
Warning::
https://web.mit.edu/themes/mit/assets/js/main.js?t...: 184 KB unused (50%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-M8M...: 67 KB unused (57%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 252 KB
252 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://web.mit.edu/themes/mit/assets/js/main.js?t5zcu8367 KB184 KB (50%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-M8MGV4P119 KB67 KB (57%)✓3rd
https://web.mit.edu/themes/mit/assets/js/modernizr.js?t5zcu86 KB—✓3rd
https://web.mit.edu/themes/mit/assets/js/SpotlightsExploreMoreLink.24ae33fb185156d3181a.js2 KB—✓3rd
https://web.mit.edu/themes/mit/assets/js/SpotlightsRecent.d6a3aa4b97b1659f1bbb.js5 KB—✓3rd
https://web.mit.edu/themes/mit/assets/js/Notification.11fc703b491d61aecf97.js4 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
Page Weight Inventory
Action
29 resources · 1.3 MB · 252 KB savings available
REVIEW
29 resources · 1.3 MB · 252 KB savings available
Warning::
Page weight 283 KB over 1MB target
Got: 1.3 MB Expected: ≤ 1 MB
Info::
~252 KB of savings available
Warning::
28 third-party resources (100% of weight)
Info::
2 resources over 200KB
29 resources · 1.3 MiB · 28 third-party 282.7 KiB over 1MB target
Image 45% · 582.5 KiB
JavaScript 38% · 502.7 KiB
Font 10% · 129.4 KiB
CSS 6% · 76.7 KiB
Document 1% · 14.5 KiB
Other 0% · 897 B

~251.8 KiB of savings available

Remove unused JavaScript: 251.8 KiB
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
5 fonts (129 KB)
PASS
5 fonts (129 KB)
Info::
5 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

5

129 KB total

Render-blocking

0

of 5

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 6.1 s LCP 9.6 shttps://use.typekit.net/af/912d1e/00000000000000007735bb33/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n4&v=3l https://use.typekit.net/af/912d1e/00000000000000007735bb33/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n4&v=3 — 25.9 KiB — 24 ms — font-display: swaphttps://use.typekit.net/af/2288cb/00000000000000007735bb39/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3l https://use.typekit.net/af/2288cb/00000000000000007735bb39/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3 — 27.2 KiB — 21 ms — font-display: swaphttps://use.typekit.net/af/c002e2/00000000000000007735bb32/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3l https://use.typekit.net/af/c002e2/00000000000000007735bb32/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3 — 28.0 KiB — 26 ms — font-display: swaphttps://use.typekit.net/af/216c2e/00000000000000007735bb6a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3l https://use.typekit.net/af/216c2e/00000000000000007735bb6a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3 — 25.4 KiB — 26 ms — font-display: swaphttps://use.typekit.net/af/174ae3/00000000000000007735bb5a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3l https://use.typekit.net/af/174ae3/00000000000000007735bb5a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3 — 22.9 KiB — 26 ms — font-display: swap
l woff2 swap
Size 26 KB
Load time 24 ms
Start 802 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 27 KB
Load time 21 ms
Start 802 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 28 KB
Load time 26 ms
Start 803 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 25 KB
Load time 26 ms
Start 805 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 23 KB
Load time 26 ms
Start 806 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
29 requests over 1690ms
INFO
0 ms 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s FCP 6.1 s LCP 9.6 shttps://mit.edu// https://mit.edu/ — 302 — 167 B — 16 ms — http/1.1https://web.mit.edu/web.mit.edu/ https://web.mit.edu/ — 200 — 12.1 KiB — 186 ms — http/1.1https://web.mit.edu/themes/mit/assets/css/main.css?t5zcu8web.mit.edu/themes/mit/assets/css/m... https://web.mit.edu/themes/mit/assets/css/main.css?t5zcu8 — 200 — 75.1 KiB — 157 ms — http/1.1https://web.mit.edu/files/images/202604/MIT-Brian-Robinson-SL_0.jpgweb.mit.edu/files/images/202604/MIT... https://web.mit.edu/files/images/202604/MIT-Brian-Robinson-SL_0.jpg — 200 — 245.4 KiB — 333 ms — http/1.1https://web.mit.edu/themes/mit/assets/js/modernizr.js?t5zcu8web.mit.edu/themes/mit/assets/js/mo... https://web.mit.edu/themes/mit/assets/js/modernizr.js?t5zcu8 — 200 — 6.1 KiB — 314 ms — http/1.1https://use.typekit.net/uby1wjv.cssuse.typekit.net/uby1wjv.css https://use.typekit.net/uby1wjv.css — 200 — 1.4 KiB — 32 ms — h2https://p.typekit.net/p.css?s=1&k=uby1wjv&ht=tk&f=39496.39497.39498.39499.39500.39501.39502.39503.39504.39505.39506.39507.39508.39509&a=53548716&app=typekit&e=cssp.typekit.net/p.css?s=1&k=uby1wjv&h... https://p.typekit.net/p.css?s=1&k=uby1wjv&ht=tk&f=39496.39497.39498.39499.39500.39501.39502.39503.39504.39505.39506.39507.39508.39509&a=53548716&app=typekit&e=css — 200 — 172 B — 20 ms — h2https://web.mit.edu/themes/mit/assets/js/main.js?t5zcu8web.mit.edu/themes/mit/assets/js/ma... https://web.mit.edu/themes/mit/assets/js/main.js?t5zcu8 — 200 — 367.2 KiB — 177 ms — http/1.1https://web.mit.edu/themes/mit/assets/favicon/manifest.webmanifestweb.mit.edu/themes/mit/assets/favic... https://web.mit.edu/themes/mit/assets/favicon/manifest.webmanifest — 200 — 730 B — 326 ms — http/1.1https://www.googletagmanager.com/gtm.js?id=GTM-M8MGV4Pwww.googletagmanager.com/gtm.js?id=... https://www.googletagmanager.com/gtm.js?id=GTM-M8MGV4P — 200 — 118.9 KiB — 33 ms — h2https://web.mit.edu/spotlights-export/suggested-links/?_format=jsonweb.mit.edu/spotlights-export/sugge... https://web.mit.edu/spotlights-export/suggested-links/?_format=json — 200 — 835 B — 171 ms — http/1.1https://web.mit.edu/themes/mit/assets/js/SpotlightsExploreMoreLink.24ae33fb185156d3181a.jsweb.mit.edu/themes/mit/assets/js/Sp... https://web.mit.edu/themes/mit/assets/js/SpotlightsExploreMoreLink.24ae33fb185156d3181a.js — 200 — 1.7 KiB — 171 ms — http/1.1https://web.mit.edu/themes/mit/assets/js/SpotlightsRecent.d6a3aa4b97b1659f1bbb.jsweb.mit.edu/themes/mit/assets/js/Sp... https://web.mit.edu/themes/mit/assets/js/SpotlightsRecent.d6a3aa4b97b1659f1bbb.js — 200 — 5.2 KiB — 179 ms — http/1.1https://web.mit.edu/themes/mit/assets/js/Notification.11fc703b491d61aecf97.jsweb.mit.edu/themes/mit/assets/js/No... https://web.mit.edu/themes/mit/assets/js/Notification.11fc703b491d61aecf97.js — 200 — 3.7 KiB — 177 ms — http/1.1https://use.typekit.net/af/912d1e/00000000000000007735bb33/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n4&v=3use.typekit.net/af/912d1e/000000000... https://use.typekit.net/af/912d1e/00000000000000007735bb33/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n4&v=3 — 200 — 25.9 KiB — 24 ms — h2https://use.typekit.net/af/2288cb/00000000000000007735bb39/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3use.typekit.net/af/2288cb/000000000... https://use.typekit.net/af/2288cb/00000000000000007735bb39/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3 — 200 — 27.2 KiB — 21 ms — h2https://use.typekit.net/af/c002e2/00000000000000007735bb32/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3use.typekit.net/af/c002e2/000000000... https://use.typekit.net/af/c002e2/00000000000000007735bb32/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3 — 200 — 28.0 KiB — 26 ms — h2https://use.typekit.net/af/216c2e/00000000000000007735bb6a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3use.typekit.net/af/216c2e/000000000... https://use.typekit.net/af/216c2e/00000000000000007735bb6a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n7&v=3 — 200 — 25.4 KiB — 26 ms — h2https://use.typekit.net/af/174ae3/00000000000000007735bb5a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3use.typekit.net/af/174ae3/000000000... https://use.typekit.net/af/174ae3/00000000000000007735bb5a/31/l?primer=7cdcb44be4a7db8877ffa5c0007b8dd865b3bbc383831fe2ea177f62257a9191&fvd=n5&v=3 — 200 — 22.9 KiB — 26 ms — h2https://web.mit.edu/spotlights-export/recent/?_format=jsonweb.mit.edu/spotlights-export/recen... https://web.mit.edu/spotlights-export/recent/?_format=json — 200 — 1.0 KiB — 171 ms — http/1.1https://web.mit.edu/spotlights-export/announcements/?_format=jsonweb.mit.edu/spotlights-export/annou... https://web.mit.edu/spotlights-export/announcements/?_format=json — 200 — 565 B — 157 ms — http/1.1https://web.mit.edu/spotlights-export/recent/?_format=jsonweb.mit.edu/spotlights-export/recen... https://web.mit.edu/spotlights-export/recent/?_format=json — 200 — 0 B — 159 ms — http/1.1https://web.mit.edu/themes/mit/assets/favicon/favicon.svgweb.mit.edu/themes/mit/assets/favic... https://web.mit.edu/themes/mit/assets/favicon/favicon.svg — 200 — 987 B — 153 ms — http/1.1https://web.mit.edu/files/images/202604/Blue-Economy_02dw.jpgweb.mit.edu/files/images/202604/Blu... https://web.mit.edu/files/images/202604/Blue-Economy_02dw.jpg — 200 — 37.0 KiB — 169 ms — http/1.1https://web.mit.edu/files/images/202604/MIT-1S1-Energy-SLsub.jpgweb.mit.edu/files/images/202604/MIT... https://web.mit.edu/files/images/202604/MIT-1S1-Energy-SLsub.jpg — 200 — 62.2 KiB — 162 ms — http/1.1https://web.mit.edu/files/images/202604/artemis-ii-launch-nasa-00_dw.jpgweb.mit.edu/files/images/202604/art... https://web.mit.edu/files/images/202604/artemis-ii-launch-nasa-00_dw.jpg — 200 — 45.2 KiB — 180 ms — http/1.1https://web.mit.edu/files/images/202603/MIT-Hand-Tracker-SLsub.jpgweb.mit.edu/files/images/202603/MIT... https://web.mit.edu/files/images/202603/MIT-Hand-Tracker-SLsub.jpg — 200 — 48.6 KiB — 168 ms — http/1.1https://web.mit.edu/files/images/202505/Final-Thumbnails-Filehm.jpgweb.mit.edu/files/images/202505/Fin... https://web.mit.edu/files/images/202505/Final-Thumbnails-Filehm.jpg — 200 — 58.6 KiB — 322 ms — http/1.1https://web.mit.edu/files/images/202505/MIT-This-is-MIT-SLsub_0_0.jpgweb.mit.edu/files/images/202505/MIT... https://web.mit.edu/files/images/202505/MIT-This-is-MIT-SLsub_0_0.jpg — 200 — 84.6 KiB — 328 ms — http/1.1
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 1765ms · 573KB · $23/mo
web.mit.edu
Other
Costly
Execution 754ms
Transfer 367 KB
Unused 50%
Monthly Cost $10/mo
web.mit.edu
Other
Costly
Execution 369ms
Transfer 12 KB
Monthly Cost $5/mo
Unattributable
Other
Costly
Execution 295ms
Transfer 0 KB
Monthly Cost $4/mo
Google Tag Manager
Tag Manager
Costly
Execution 275ms
Transfer 119 KB
Unused 57%
Monthly Cost $4/mo
web.mit.edu
Other
Optional
Execution 72ms
Transfer 75 KB
Monthly Cost $1/mo

These scripts may cost more than they're worth

  • web.mit.edu adds 754ms and costs ~$10/month
  • web.mit.edu adds 369ms and costs ~$5/month
  • Unattributable adds 295ms and costs ~$4/month
  • Google Tag Manager adds 275ms and costs ~$4/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

web.mit.edu takes 754ms 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

50% of web.mit.edu'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

web.mit.edu takes 369ms 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 295ms 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 275ms 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

57% of Google Tag Manager'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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