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Performance

· 17 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
68
GRADE
D
FIX
5
REVIEW
1
PASS
9
INFO
2

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

Mobile Desktop
38
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
74
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.

13.35 s

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

24.95 s

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

694 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.

13.35 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.

24.98 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
9 PASS 1 REVIEW 5 FIX
F
Page Weight Budget
Action
4.4 MB transferred, 136 requests
FIX
4.4 MB transferred, 136 requests
Critical::
Page weighs 10.2 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 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::
136 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
4.4 MB / 2.4 MB budget
4.4 MB transferred
136 requests
0.93 g CO2 per page load
JavaScript 2.1 MiB 48%
Images 2.0 MiB 44%
CSS 173.2 KiB 4%
Fonts 83.0 KiB 2%
Other 36.1 KiB 1%
Other 30.1 KiB 1%
HTML 15.6 KiB 0%
Other 1.7 KiB 0%
Other 809 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
97% third-party, 0 ms blocking
FIX
97% third-party, 0 ms blocking
Warning::
Third-party code accounts for 97% of page weight (4.3 MiB of 4.4 MiB)
Info::
Third-party blocking time is low (0 ms)
97%
First-party Third-party
F
JS Execution Cost
Action
3933ms total JS execution
FIX
3933ms total JS execution
Warning::
Unattributable: 1658ms CPU time
Warning::
https://substackcdn.com/bundle/static/js/11383.633...: 1311ms CPU time
Warning::
https://substack.com/: 349ms CPU time
Warning::
https://substackcdn.com/bundle/static/js/async/505...: 312ms CPU time
Info::
https://substack.com/cdn-cgi/challenge-platform/sc...: 232ms CPU time
Warning::
Third-party scripts: 3352ms (85% of total)

Main Thread Breakdown

Script Evaluation 56% Other 28% Style & Layout 9%
ScriptTotalScriptingParse/CompileParty
Unattributable1658ms674ms0ms3rd
https://substackcdn.com/bundle/static/js/11383.633fb0f1.js1311ms1267ms1ms3rd
https://substack.com/349ms52ms6ms1st
https://substackcdn.com/bundle/static/js/async/50551.430c8de0.js312ms121ms21ms3rd
https://substack.com/cdn-cgi/challenge-platform/scripts/jsd/main.js232ms219ms3ms1st
https://substackcdn.com/bundle/static/js/reader2.15dc9b3f.js72ms4ms7ms3rd
F
JS Bundles
Action
99 scripts, 1022 KB unused
FIX
99 scripts, 1022 KB unused
Warning::
https://substackcdn.com/bundle/static/js/reader2.1...: 376 KB unused (59%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://substackcdn.com/bundle/static/js/63549.4f4...: 241 KB unused (79%)
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/16652.973...: 52 KB unused (85%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://substackcdn.com/bundle/static/js/11383.633...: 42 KB unused (71%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 1022 KB
1022 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://substackcdn.com/bundle/static/js/reader2.15dc9b3f.js636 KB376 KB (59%)✓3rd
https://substackcdn.com/bundle/static/js/63549.4f403a53.js312 KB241 KB (79%)✓3rd
https://substackcdn.com/bundle/static/js/async/50551.430c8de0.js95 KB82 KB (86%)✓3rd
https://substackcdn.com/bundle/static/js/16652.97367451.js62 KB52 KB (85%)✓3rd
https://substackcdn.com/bundle/static/js/11383.633fb0f1.js60 KB42 KB (71%)✓3rd
https://substackcdn.com/bundle/static/js/96869.6178d54d.js38 KB38 KB (100%)✓3rd
https://substackcdn.com/bundle/static/js/27438.ac9b0f39.js42 KB33 KB (80%)✓3rd
https://substackcdn.com/bundle/static/js/31345.0eab9ae9.js44 KB32 KB (74%)✓3rd
https://substackcdn.com/bundle/static/js/11578.9eef8573.js29 KB29 KB (99%)✓3rd
https://substackcdn.com/bundle/static/js/61861.a8ba0c8d.js38 KB26 KB (70%)✓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

F
Page Weight Inventory
Action
136 resources · 4.4 MB · 1022 KB savings available
FIX
136 resources · 4.4 MB · 1022 KB savings available
Warning::
Page weight 3.4 MB over 1MB target
Got: 4.4 MB Expected: ≤ 1 MB
Info::
~1022 KB of savings available
Warning::
127 third-party resources (97% of weight)
Info::
3 resources over 200KB
136 resources · 4.4 MiB · 127 third-party 3.4 MiB over 1MB target
JavaScript 48% · 2.1 MiB
Image 45% · 2.0 MiB
CSS 4% · 173.2 KiB
Font 2% · 83.0 KiB
Other 1% · 39.1 KiB
Document 0% · 15.6 KiB

~1021.9 KiB of savings available

Remove unused JavaScript: 1021.9 KiB
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
14 images, 0 KB saveable
PASS
14 images, 0 KB saveable
Warning::
https://substackcdn.com/image/fetch/$s_!iJMe!,w_39... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://substackcdn.com/image/fetch/$s_!DzDi!,w_24... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
14 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
2 fonts (83 KB)
PASS
2 fonts (83 KB)
Info::
2 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

2

83 KB total

Render-blocking

0

of 2

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 13.4 s LCP 24.9 shttps://substackcdn.com/fonts/Cahuenga-SemiBold.woff2Cahuenga-SemiBold.woff2 https://substackcdn.com/fonts/Cahuenga-SemiBold.woff2 — 69.5 KiB — 56 ms — font-display: swaphttps://fonts.gstatic.com/s/spectral/v13/rnCr-xNNww_2s0amA9M5knjsS_ul.woff2rnCr-xNNww_2s0amA9M5knj... https://fonts.gstatic.com/s/spectral/v13/rnCr-xNNww_2s0amA9M5knjsS_ul.woff2 — 13.5 KiB — 116 ms — font-display: swap
Cahuenga-SemiBold.woff2 woff2 swap
Size 69 KB
Load time 56 ms
Start 475 ms
Risk FOUT — text flashes from fallback to web font
rnCr-xNNww_2s0amA9M5knj... woff2 swap
Size 13 KB
Load time 116 ms
Start 1.9 s
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
136 requests over 2398ms
INFO
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 13.4 s LCP 24.9 shttps://substack.com// https://substack.com/ — 200 — 15.6 KiB — 206 ms — h2https://substack.com/bundle/theme/reader2.0a4d04ea913496ef342f.css/bundle/theme/reader2.0a4d04ea91349... https://substack.com/bundle/theme/reader2.0a4d04ea913496ef342f.css — 200 — 53.5 KiB — 59 ms — h2https://substackcdn.com/bundle/static/css/382.b89e30f6.csssubstackcdn.com/bundle/static/css/3... https://substackcdn.com/bundle/static/css/382.b89e30f6.css — 200 — 21.3 KiB — 96 ms — h2https://substackcdn.com/bundle/static/css/56938.bc21f918.csssubstackcdn.com/bundle/static/css/5... https://substackcdn.com/bundle/static/css/56938.bc21f918.css — 200 — 4.8 KiB — 98 ms — h2https://substackcdn.com/bundle/static/css/54636.1791a1b4.csssubstackcdn.com/bundle/static/css/5... https://substackcdn.com/bundle/static/css/54636.1791a1b4.css — 200 — 4.4 KiB — 99 ms — h2https://substackcdn.com/bundle/static/css/82434.e48dc20a.csssubstackcdn.com/bundle/static/css/8... https://substackcdn.com/bundle/static/css/82434.e48dc20a.css — 200 — 3.7 KiB — 100 ms — h2https://substackcdn.com/bundle/static/css/11383.375bfa89.csssubstackcdn.com/bundle/static/css/1... https://substackcdn.com/bundle/static/css/11383.375bfa89.css — 200 — 32.5 KiB — 114 ms — h2https://substackcdn.com/bundle/static/css/31345.0b820313.csssubstackcdn.com/bundle/static/css/3... https://substackcdn.com/bundle/static/css/31345.0b820313.css — 200 — 7.1 KiB — 115 ms — h2https://substackcdn.com/bundle/static/css/95680.261114a6.csssubstackcdn.com/bundle/static/css/9... https://substackcdn.com/bundle/static/css/95680.261114a6.css — 200 — 3.8 KiB — 116 ms — h2https://substackcdn.com/bundle/static/css/reader2.66f17a7b.csssubstackcdn.com/bundle/static/css/r... https://substackcdn.com/bundle/static/css/reader2.66f17a7b.css — 200 — 33.5 KiB — 123 ms — h2https://substackcdn.com/bundle/static/css/11578.4a18acbd.csssubstackcdn.com/bundle/static/css/1... https://substackcdn.com/bundle/static/css/11578.4a18acbd.css — 200 — 6.6 KiB — 127 ms — h2https://substackcdn.com/bundle/static/js/lib-router.cdba740d.jssubstackcdn.com/bundle/static/js/li... https://substackcdn.com/bundle/static/js/lib-router.cdba740d.js — 200 — 25.9 KiB — 280 ms — h2https://substackcdn.com/bundle/static/js/92520.c9b58344.jssubstackcdn.com/bundle/static/js/92... https://substackcdn.com/bundle/static/js/92520.c9b58344.js — 200 — 59.5 KiB — 287 ms — h2https://substackcdn.com/bundle/static/js/43876.76bb2505.jssubstackcdn.com/bundle/static/js/43... https://substackcdn.com/bundle/static/js/43876.76bb2505.js — 200 — 2.7 KiB — 266 ms — h2https://substackcdn.com/bundle/static/js/75058.d532797c.jssubstackcdn.com/bundle/static/js/75... https://substackcdn.com/bundle/static/js/75058.d532797c.js — 200 — 2.5 KiB — 266 ms — h2https://substackcdn.com/bundle/static/js/61861.a8ba0c8d.jssubstackcdn.com/bundle/static/js/61... https://substackcdn.com/bundle/static/js/61861.a8ba0c8d.js — 200 — 38.3 KiB — 271 ms — h2https://substackcdn.com/bundle/static/js/85929.cb5a349d.jssubstackcdn.com/bundle/static/js/85... https://substackcdn.com/bundle/static/js/85929.cb5a349d.js — 200 — 5.5 KiB — 268 ms — h2https://substackcdn.com/bundle/static/js/70780.a701d539.jssubstackcdn.com/bundle/static/js/70... https://substackcdn.com/bundle/static/js/70780.a701d539.js — 200 — 21.8 KiB — 271 ms — h2https://substackcdn.com/bundle/static/js/83988.f143cb59.jssubstackcdn.com/bundle/static/js/83... https://substackcdn.com/bundle/static/js/83988.f143cb59.js — 200 — 8.4 KiB — 269 ms — h2https://substackcdn.com/bundle/static/js/23553.b588b8d5.jssubstackcdn.com/bundle/static/js/23... https://substackcdn.com/bundle/static/js/23553.b588b8d5.js — 200 — 4.9 KiB — 269 ms — h2https://substackcdn.com/bundle/static/js/63549.4f403a53.jssubstackcdn.com/bundle/static/js/63... https://substackcdn.com/bundle/static/js/63549.4f403a53.js — 200 — 312.0 KiB — 308 ms — h2https://substackcdn.com/bundle/static/js/76976.ae6d6036.jssubstackcdn.com/bundle/static/js/76... https://substackcdn.com/bundle/static/js/76976.ae6d6036.js — 200 — 7.7 KiB — 260 ms — h2https://substackcdn.com/bundle/static/js/19971.a782ee5c.jssubstackcdn.com/bundle/static/js/19... https://substackcdn.com/bundle/static/js/19971.a782ee5c.js — 200 — 9.6 KiB — 260 ms — h2https://substackcdn.com/bundle/static/js/27994.915bf6b8.jssubstackcdn.com/bundle/static/js/27... https://substackcdn.com/bundle/static/js/27994.915bf6b8.js — 200 — 2.7 KiB — 260 ms — h2https://substackcdn.com/bundle/static/js/67562.de8bf319.jssubstackcdn.com/bundle/static/js/67... https://substackcdn.com/bundle/static/js/67562.de8bf319.js — 200 — 4.8 KiB — 261 ms — h2https://substackcdn.com/bundle/static/js/16932.a8d9a03d.jssubstackcdn.com/bundle/static/js/16... https://substackcdn.com/bundle/static/js/16932.a8d9a03d.js — 200 — 7.4 KiB — 261 ms — h2https://substackcdn.com/bundle/static/js/49271.7ae0ad97.jssubstackcdn.com/bundle/static/js/49... https://substackcdn.com/bundle/static/js/49271.7ae0ad97.js — 200 — 13.1 KiB — 261 ms — h2https://substackcdn.com/bundle/static/js/10107.aa2b5d19.jssubstackcdn.com/bundle/static/js/10... https://substackcdn.com/bundle/static/js/10107.aa2b5d19.js — 200 — 14.9 KiB — 262 ms — h2https://substackcdn.com/bundle/static/js/49907.a127f490.jssubstackcdn.com/bundle/static/js/49... https://substackcdn.com/bundle/static/js/49907.a127f490.js — 200 — 3.4 KiB — 263 ms — h2https://substackcdn.com/bundle/static/js/39010.9c0f5490.jssubstackcdn.com/bundle/static/js/39... https://substackcdn.com/bundle/static/js/39010.9c0f5490.js — 200 — 3.5 KiB — 257 ms — h2
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 13.4 s LCP 24.9 shttps://substack.com// https://substack.com/ — 200 — 15.6 KiB — 206 ms — h2https://substack.com/bundle/theme/reader2.0a4d04ea913496ef342f.css/bundle/theme/reader2.0a4d04ea91349... https://substack.com/bundle/theme/reader2.0a4d04ea913496ef342f.css — 200 — 53.5 KiB — 59 ms — h2https://substackcdn.com/bundle/static/css/382.b89e30f6.csssubstackcdn.com/bundle/static/css/3... https://substackcdn.com/bundle/static/css/382.b89e30f6.css — 200 — 21.3 KiB — 96 ms — h2https://substackcdn.com/bundle/static/css/56938.bc21f918.csssubstackcdn.com/bundle/static/css/5... https://substackcdn.com/bundle/static/css/56938.bc21f918.css — 200 — 4.8 KiB — 98 ms — h2https://substackcdn.com/bundle/static/css/54636.1791a1b4.csssubstackcdn.com/bundle/static/css/5... https://substackcdn.com/bundle/static/css/54636.1791a1b4.css — 200 — 4.4 KiB — 99 ms — h2https://substackcdn.com/bundle/static/css/82434.e48dc20a.csssubstackcdn.com/bundle/static/css/8... https://substackcdn.com/bundle/static/css/82434.e48dc20a.css — 200 — 3.7 KiB — 100 ms — h2https://substackcdn.com/bundle/static/css/11383.375bfa89.csssubstackcdn.com/bundle/static/css/1... https://substackcdn.com/bundle/static/css/11383.375bfa89.css — 200 — 32.5 KiB — 114 ms — h2https://substackcdn.com/bundle/static/css/31345.0b820313.csssubstackcdn.com/bundle/static/css/3... https://substackcdn.com/bundle/static/css/31345.0b820313.css — 200 — 7.1 KiB — 115 ms — h2https://substackcdn.com/bundle/static/css/95680.261114a6.csssubstackcdn.com/bundle/static/css/9... https://substackcdn.com/bundle/static/css/95680.261114a6.css — 200 — 3.8 KiB — 116 ms — h2https://substackcdn.com/bundle/static/css/reader2.66f17a7b.csssubstackcdn.com/bundle/static/css/r... https://substackcdn.com/bundle/static/css/reader2.66f17a7b.css — 200 — 33.5 KiB — 123 ms — h2https://substackcdn.com/bundle/static/css/11578.4a18acbd.csssubstackcdn.com/bundle/static/css/1... https://substackcdn.com/bundle/static/css/11578.4a18acbd.css — 200 — 6.6 KiB — 127 ms — h2https://substackcdn.com/bundle/static/js/lib-router.cdba740d.jssubstackcdn.com/bundle/static/js/li... https://substackcdn.com/bundle/static/js/lib-router.cdba740d.js — 200 — 25.9 KiB — 280 ms — h2https://substackcdn.com/bundle/static/js/92520.c9b58344.jssubstackcdn.com/bundle/static/js/92... https://substackcdn.com/bundle/static/js/92520.c9b58344.js — 200 — 59.5 KiB — 287 ms — h2https://substackcdn.com/bundle/static/js/43876.76bb2505.jssubstackcdn.com/bundle/static/js/43... https://substackcdn.com/bundle/static/js/43876.76bb2505.js — 200 — 2.7 KiB — 266 ms — h2https://substackcdn.com/bundle/static/js/75058.d532797c.jssubstackcdn.com/bundle/static/js/75... https://substackcdn.com/bundle/static/js/75058.d532797c.js — 200 — 2.5 KiB — 266 ms — 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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
85% of JavaScript execution is third-party
First-party Third-party 3352ms · 792KB · $11/mo
Unattributable
Other
Costly
Execution 1658ms
Transfer 0 KB
Monthly Cost $5/mo
substackcdn.com
Other
Costly
Execution 1311ms
Transfer 60 KB
Unused 71%
Monthly Cost $4/mo
substackcdn.com
Other
Costly
Execution 312ms
Transfer 95 KB
Unused 86%
Monthly Cost $1/mo
substackcdn.com
Other
Optional
Execution 72ms
Transfer 636 KB
Unused 59%
Monthly Cost $0/mo

These scripts may cost more than they're worth

  • Unattributable adds 1658ms and costs ~$5/month
  • substackcdn.com adds 1311ms and costs ~$4/month
  • substackcdn.com adds 312ms and costs ~$1/month

85% 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 1658ms 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 1311ms 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

71% 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 312ms 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

59% 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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