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Performance

· 16 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
72
GRADE
C
FIX
4
REVIEW
1
PASS
9
INFO
2

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

Mobile Desktop
55
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
70
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.
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.

14.14 s

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

25.33 s

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

48 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.

14.14 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.

25.37 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
16
9 PASS 1 REVIEW 4 FIX
F
Page Weight Budget
Action
4.5 MB transferred, 141 requests
FIX
4.5 MB transferred, 141 requests
Critical::
Page weighs 10.1 MB (4.5 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 2.0 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 2.0 MB
Warning::
141 HTTP requests — consider bundling or reducing
Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.
Info::
Estimated 0.93 g CO2 per page load
4.5 MB / 2.4 MB budget
4.5 MB transferred
141 requests
0.93 g CO2 per page load
JavaScript 2.1 MiB 48%
Images 2.0 MiB 46%
CSS 167.5 KiB 4%
Fonts 69.5 KiB 2%
Other 31.2 KiB 1%
Other 18.6 KiB 0%
HTML 15.4 KiB 0%
Other 1.7 KiB 0%
Other 816 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 (4.5 MiB of 4.5 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
F
JS Execution Cost
Action
4076ms total JS execution
FIX
4076ms total JS execution
Warning::
https://substackcdn.com/bundle/static/js/77496.a03...: 2119ms CPU time
Warning::
Unattributable: 1150ms CPU time
Warning::
https://substack.com/cdn-cgi/challenge-platform/sc...: 257ms CPU time
Warning::
https://substack.com/: 254ms CPU time
Info::
https://substackcdn.com/bundle/static/js/async/505...: 221ms CPU time
Warning::
Third-party scripts: 4076ms (100% of total)

Main Thread Breakdown

Script Evaluation 57% Other 31% Style & Layout 6%
ScriptTotalScriptingParse/CompileParty
https://substackcdn.com/bundle/static/js/77496.a030b7b6.js2119ms2027ms1ms3rd
Unattributable1150ms31ms0ms3rd
https://substack.com/cdn-cgi/challenge-platform/scripts/jsd/main.js257ms247ms2ms3rd
https://substack.com/254ms39ms6ms3rd
https://substackcdn.com/bundle/static/js/async/50551.9f31dfda.js221ms109ms30ms3rd
https://substackcdn.com/bundle/static/js/reader2.dd4aee9b.js75ms4ms6ms3rd
F
JS Bundles
Action
102 scripts, 982 KB unused
FIX
102 scripts, 982 KB unused
Warning::
https://substackcdn.com/bundle/static/js/reader2.d...: 348 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: 982 KB
982 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://substackcdn.com/bundle/static/js/reader2.dd4aee9b.js621 KB348 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 KB35 KB (80%)✓3rd
https://substackcdn.com/bundle/static/js/96924.6b3177b3.js44 KB33 KB (75%)✓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
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
Image Optimization
16 images, 0 KB saveable
PASS
16 images, 0 KB saveable
Warning::
https://substackcdn.com/image/fetch/$s_!DzDi!,w_24... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://substackcdn.com/image/fetch/$s_!iJMe!,w_39... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
16 images 2.0 MB
0 oversized -0 KB
1 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

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 2.0 s 4.0 s 6.0 s 8.0 s 10.0 s 12.0 s 14.0 s 16.0 s 18.0 s 20.0 s 22.0 s 24.0 s 26.0 s FCP 14.1 s LCP 25.3 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
2 hints, 0 missing preconnects
PASS
2 hints, 0 missing preconnects
Info::
Page uses 2 resource hint(s)

Current Resource Hints

preconnect
1
preload
1
dns-prefetch
—
prefetch
—

2 resource hints configured

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+
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
141 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 4076ms · 814KB · $13/mo
substackcdn.com
Other
Costly
Execution 2119ms
Transfer 82 KB
Unused 75%
Monthly Cost $7/mo
Unattributable
Other
Costly
Execution 1150ms
Transfer 0 KB
Monthly Cost $4/mo
substack.com
Other
Costly
Execution 257ms
Transfer 0 KB
Monthly Cost $1/mo
substack.com
Other
Costly
Execution 254ms
Transfer 15 KB
Monthly Cost $1/mo
substackcdn.com
Other
Costly
Execution 221ms
Transfer 95 KB
Unused 86%
Monthly Cost $1/mo
substackcdn.com
Other
Optional
Execution 75ms
Transfer 621 KB
Unused 56%
Monthly Cost $0/mo

These scripts may cost more than they're worth

  • substackcdn.com adds 2119ms and costs ~$7/month
  • Unattributable adds 1150ms and costs ~$4/month
  • substack.com adds 257ms and costs ~$1/month
  • substack.com adds 254ms and costs ~$1/month
  • substackcdn.com adds 221ms 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

substackcdn.com takes 2119ms 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

Unattributable takes 1150ms 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

substack.com takes 257ms 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

substack.com takes 254ms 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 221ms 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

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

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

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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