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Performance

· 14 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
72
GRADE
C
FIX
3
REVIEW
3
PASS
6
INFO
2

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

Mobile Desktop
47
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
75
Accessibility Measures how accessible the page is for users with disabilities. Checks color contrast, ARIA labels, and semantic HTML.
77
Best Practices Checks for modern web development best practices including HTTPS, no console errors, and secure JavaScript.
85
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.

1.59 s

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

5.32 s

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

477 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.

2.04 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.

5.32 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
6 PASS 3 REVIEW 3 FIX
F
Page Weight Budget
Action
4.4 MB transferred, 150 requests
FIX
4.4 MB transferred, 150 requests
Critical::
Page weighs 10.4 MB (4.4 MB transferred)
Warning::
JavaScript is 2.1 MB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 2.1 MB
Warning::
Images are 1.9 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 1.9 MB
Warning::
150 HTTP requests — consider bundling or reducing
Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.
Info::
Estimated 0.92 g CO2 per page load
4.4 MB / 2.4 MB budget
4.4 MB transferred
150 requests
0.92 g CO2 per page load
JavaScript 2.1 MiB 48%
Images 1.9 MiB 43%
CSS 162.1 KiB 4%
Other 105.8 KiB 2%
Fonts 69.5 KiB 2%
Other 31.0 KiB 1%
HTML 16.5 KiB 0%
Other 1.7 KiB 0%
Other 1014 B 0%
0 2.4 MB 4.9 MB
Over 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

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

Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.

Why this matters

High HTTP request count — bundling, sprite-ing, and HTTP/2 server push can reduce per-request overhead.

Source: web.dev

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 (4.4 MiB of 4.4 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
F
JS Bundles
Action
102 scripts, 975 KB unused
FIX
102 scripts, 975 KB unused
Warning::
https://substackcdn.com/bundle/static/js/reader2.d...: 344 KB unused (56%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://substackcdn.com/bundle/static/js/51606.9e5...: 236 KB unused (78%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://substackcdn.com/bundle/static/js/async/505...: 82 KB unused (86%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://substackcdn.com/bundle/static/js/77496.a03...: 61 KB unused (75%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://substackcdn.com/bundle/static/js/96869.cf1...: 38 KB unused (100%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 975 KB
975 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://substackcdn.com/bundle/static/js/reader2.dd4aee9b.js621 KB344 KB (56%)✓3rd
https://substackcdn.com/bundle/static/js/51606.9e5d2f4f.js306 KB236 KB (78%)✓3rd
https://substackcdn.com/bundle/static/js/async/50551.9f31dfda.js95 KB82 KB (86%)✓3rd
https://substackcdn.com/bundle/static/js/77496.a030b7b6.js82 KB61 KB (75%)✓3rd
https://substackcdn.com/bundle/static/js/96869.cf1a6316.js38 KB38 KB (100%)✓3rd
https://substackcdn.com/bundle/static/js/30440.3a078123.js45 KB36 KB (80%)✓3rd
https://substackcdn.com/bundle/static/js/96924.6b3177b3.js44 KB32 KB (72%)✓3rd
https://substackcdn.com/bundle/static/js/97993.36609a05.js28 KB27 KB (99%)✓3rd
https://substackcdn.com/bundle/static/js/62744.d65af920.js36 KB27 KB (77%)✓3rd
https://substackcdn.com/bundle/static/js/lib-router.faae56b7.js26 KB24 KB (95%)✓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

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

B
Image Optimization
23 images, 0 KB saveable
REVIEW
23 images, 0 KB saveable
Warning::
https://substackcdn.com/image/fetch/$s_!u9e1!,w_30... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://substackcdn.com/image/fetch/$s_!DzDi!,w_17... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
23 images 1.9 MB
0 oversized -0 KB
2 legacy format
2 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

C
JS Execution Cost
Action
1729ms total JS execution
REVIEW
1729ms total JS execution
Warning::
Unattributable: 533ms CPU time
Warning::
https://substackcdn.com/bundle/static/js/77496.a03...: 531ms CPU time
Warning::
https://substackcdn.com/bundle/static/js/reader2.d...: 270ms CPU time
Info::
https://substack.com/: 181ms CPU time
Info::
https://substackcdn.com/bundle/static/js/async/505...: 108ms CPU time
Warning::
Third-party scripts: 1729ms (100% of total)

Main Thread Breakdown

Script Evaluation 56% Other 31% Style & Layout 6% Garbage Collection 4%
ScriptTotalScriptingParse/CompileParty
Unattributable533ms102ms0ms3rd
https://substackcdn.com/bundle/static/js/77496.a030b7b6.js531ms506ms0ms3rd
https://substackcdn.com/bundle/static/js/reader2.dd4aee9b.js270ms203ms4ms3rd
https://substack.com/181ms17ms2ms3rd
https://substackcdn.com/bundle/static/js/async/50551.9f31dfda.js108ms92ms10ms3rd
https://substack.com/cdn-cgi/challenge-platform/scripts/jsd/main.js107ms103ms1ms3rd
B
HTTP Caching
no-cache
REVIEW
no-cache
Info::
Cache-Control header is set
Got: no-cache
Info::
No ETag or Last-Modified header
Conditional requests (304 Not Modified) are not possible without validators.
Cache-Control

no-cache

DirectiveValueMeaning
no-cache—Cache but always revalidate first
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
1 fonts (69 KB)
PASS
1 fonts (69 KB)
Info::
1 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

1

69 KB total

Render-blocking

0

of 1

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 4.0 s 5.0 s FCP 1.6 s LCP 5.3 shttps://substackcdn.com/fonts/Cahuenga-SemiBold.woff2Cahuenga-SemiBold.woff2 https://substackcdn.com/fonts/Cahuenga-SemiBold.woff2 — 69.5 KiB — 0 ms — font-display: swap
Cahuenga-SemiBold.woff2 woff2 swap
Size 69 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+
Green Hosting
Whether the site is served from green-energy infrastructure
PASS

Green Hosting

This site is hosted on green energy infrastructure

Provider: Cloudflare

Network Waterfall
150 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
100% of JavaScript execution is third-party
First-party Third-party 1729ms · 815KB · $6/mo
Unattributable
Other
Costly
Execution 533ms
Transfer 0 KB
Monthly Cost $2/mo
substackcdn.com
Other
Costly
Execution 531ms
Transfer 82 KB
Unused 75%
Monthly Cost $2/mo
substackcdn.com
Other
Costly
Execution 270ms
Transfer 621 KB
Unused 56%
Monthly Cost $1/mo
substack.com
Other
Optional
Execution 181ms
Transfer 16 KB
Monthly Cost $1/mo
substackcdn.com
Other
Optional
Execution 108ms
Transfer 95 KB
Unused 86%
Monthly Cost $0/mo
substack.com
Other
Optional
Execution 107ms
Transfer 0 KB
Monthly Cost $0/mo

These scripts may cost more than they're worth

  • Unattributable adds 533ms and costs ~$2/month
  • substackcdn.com adds 531ms and costs ~$2/month
  • substackcdn.com adds 270ms and costs ~$1/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

Unattributable takes 533ms 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

substackcdn.com takes 531ms 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

75% of substackcdn.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

substackcdn.com takes 270ms 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

56% of substackcdn.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

86% of substackcdn.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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