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Performance

· 14 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
86
GRADE
B
FIX
1
REVIEW
4
PASS
7
INFO
2

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

Mobile Desktop
81
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
89
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.

1.90 s

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

1.97 s

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

18 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.

1.91 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.

2.57 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
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.
Info::
ETag present — enables conditional (304) responses
Info::
Last-Modified header present

No Cache-Control header

Adding a Cache-Control header can significantly improve repeat-visit performance.

B
Page Weight Budget
1.4 MB transferred, 28 requests
REVIEW
1.4 MB transferred, 28 requests
Info::
Page weighs 2.0 MB (1.4 MB transferred)
Warning::
JavaScript is 502 KB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 502 KB
Info::
28 HTTP requests
Info::
Estimated 0.29 g CO2 per page load
1.4 MB / 2.4 MB budget
1.4 MB transferred
28 requests
0.29 g CO2 per page load
Images 709.5 KiB 49%
JavaScript 501.5 KiB 35%
Fonts 129.4 KiB 9%
CSS 76.7 KiB 5%
HTML 12.2 KiB 1%
Other 2.5 KiB 0%
Other 988 B 0%
Other 731 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://www.mit.edu/files/images/202604/MIT-PhytoF... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
7 images 709 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 Bundles
Action
6 scripts, 251 KB unused
REVIEW
6 scripts, 251 KB unused
Warning::
https://www.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: 251 KB
251 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.mit.edu/themes/mit/assets/js/main.js?t5zcu8367 KB184 KB (50%)✓1st
https://www.googletagmanager.com/gtm.js?id=GTM-M8MGV4P118 KB67 KB (57%)✓3rd
https://www.mit.edu/themes/mit/assets/js/modernizr.js?t5zcu86 KB—✓1st
https://www.mit.edu/themes/mit/assets/js/SpotlightsExploreMoreLink.24ae33fb185156d3181a.js2 KB—✓1st
https://www.mit.edu/themes/mit/assets/js/SpotlightsRecent.d6a3aa4b97b1659f1bbb.js5 KB—✓1st
https://www.mit.edu/themes/mit/assets/js/Notification.11fc703b491d61aecf97.js4 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

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

Green Hosting

No green hosting detected

A+
Third-Party Impact
17% third-party, 0 ms blocking
PASS
17% third-party, 0 ms blocking
Info::
Third-party code accounts for 17% of page weight (248.7 KiB of 1.4 MiB)
Info::
Third-party blocking time is low (0 ms)
83%
17%
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+
JS Execution Cost
366ms total JS execution
PASS
366ms total JS execution
Info::
https://www.mit.edu/themes/mit/assets/js/main.js?t...: 103ms CPU time
Info::
https://www.mit.edu/: 101ms CPU time
Info::
Unattributable: 57ms CPU time
Info::
https://www.mit.edu/themes/mit/assets/js/modernizr...: 53ms CPU time
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-M8M...: 52ms CPU time
Info::
Third-party scripts: 109ms (30% of total)

Main Thread Breakdown

Script Evaluation 36% Other 30% Style & Layout 21% Script Parsing & Compilation 6% Rendering 4% Parse HTML & CSS 3%
ScriptTotalScriptingParse/CompileParty
https://www.mit.edu/themes/mit/assets/js/main.js?t5zcu8103ms82ms12ms1st
https://www.mit.edu/101ms3ms0ms1st
Unattributable57ms2ms0ms3rd
https://www.mit.edu/themes/mit/assets/js/modernizr.js?t5zcu853ms4ms0ms1st
https://www.googletagmanager.com/gtm.js?id=GTM-M8MGV4P52ms38ms11ms3rd
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 500 ms 1.0 s 1.5 s 2.0 s FCP 1.9 s LCP 2.0 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 — 0 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 — 0 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 — 0 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 — 0 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 — 0 ms — font-display: swap
l woff2 swap
Size 26 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 27 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 28 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 25 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
l woff2 swap
Size 23 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
Network Waterfall
28 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
30% of JavaScript execution is third-party
First-party Third-party 109ms · 118KB · $1/mo
Unattributable
Other
Optional
Execution 57ms
Transfer 0 KB
Monthly Cost $1/mo
Google Tag Manager
Tag Manager
Optional
Execution 52ms
Transfer 118 KB
Unused 57%
Monthly Cost $1/mo

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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