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Performance

· 15 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
79
GRADE
C
FIX
3
REVIEW
3
PASS
7
INFO
2

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

Mobile Desktop
83
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
95
Accessibility Measures how accessible the page is for users with disabilities. Checks color contrast, ARIA labels, and semantic HTML.
81
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.

1.64 s

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

1.99 s

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

40 ms

Cumulative Layout Shift Cumulative Layout Shift — measures visual stability. How much the page layout shifts during loading. Under 0.1 is good.

0.021

Speed Index Speed Index — how quickly content is visually displayed during load. Under 3.4s is good.

1.64 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.

2.04 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
15
7 PASS 3 REVIEW 3 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 (1.8 MiB of 1.8 MiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
D
Page Weight Inventory
Action
50 resources · 1.8 MB · 140 KB savings available
FIX
50 resources · 1.8 MB · 140 KB savings available
Warning::
Page weight 832 KB over 1MB target
Got: 1.8 MB Expected: ≤ 1 MB
Info::
~140 KB of savings available
Warning::
49 third-party resources (100% of weight)
Info::
1 resources over 200KB
50 resources · 1.8 MiB · 49 third-party 831.9 KiB over 1MB target
Font 56% · 1.0 MiB
Image 19% · 348.8 KiB
JavaScript 14% · 257.2 KiB
CSS 11% · 195.9 KiB
Document 1% · 12.2 KiB
Other 0% · 2.6 KiB

~140.0 KiB of savings available

Remove unused JavaScript: 140.0 KiB
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.

C
Page Weight Budget
Action
1.8 MB transferred, 50 requests
REVIEW
1.8 MB transferred, 50 requests
Info::
Page weighs 3.5 MB (1.8 MB transferred)
Info::
Fonts are 1.0 MB — consider subsetting or using system fonts
Subset fonts to include only used characters, or switch to system font stacks.
Got: 1.0 MB
Info::
50 HTTP requests
Info::
Estimated 0.38 g CO2 per page load
1.8 MB / 2.4 MB budget
1.8 MB transferred
50 requests
0.38 g CO2 per page load
Fonts 1.0 MiB 56%
Images 344.6 KiB 19%
JavaScript 257.2 KiB 14%
CSS 102.0 KiB 5%
Other 93.9 KiB 5%
Other 9.3 KiB 0%
HTML 7.3 KiB 0%
Other 2.5 KiB 0%
0 2.4 MB 4.9 MB
Approaching limit

Subset fonts to include only used characters, or switch to system font stacks.

Why this matters

Font payload is large — subset to used glyphs only, or fall back to system fonts (zero load time).

Source: web.dev font loading

C
Image Optimization
Action
13 images, 0 KB saveable
REVIEW
13 images, 0 KB saveable
Warning::
https://about.readthedocs.com/images/homepage.png is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/flask-logo.pn... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/jupyter-logo.... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/theme/img/logo-wordm... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/amd-logo.svg is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/canonical-log... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/anaconda-logo... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/acquia-logo.s... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://about.readthedocs.com/images/godot-logo.pn... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
13 images 345 KB
0 oversized -0 KB
7 legacy format
9 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

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

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

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

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

B
JS Bundles
4 scripts, 140 KB unused
REVIEW
4 scripts, 140 KB unused
Warning::
https://about.readthedocs.com/theme/js/vendor.js: 89 KB unused (77%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://about.readthedocs.com/_/static/javascript/...: 51 KB unused (39%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 140 KB
140 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://about.readthedocs.com/theme/js/vendor.js117 KB89 KB (77%)✓3rd
https://about.readthedocs.com/_/static/javascript/readthedocs-addons.js132 KB51 KB (39%)✓3rd
https://kit.fontawesome.com/53af4251af.js6 KB—✓3rd
https://about.readthedocs.com/theme/js/site.js3 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

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
466ms total JS execution
PASS
466ms total JS execution
Info::
https://about.readthedocs.com/_/static/javascript/...: 205ms CPU time
Info::
https://about.readthedocs.com/?ref=app.readthedocs...: 165ms CPU time
Info::
Unattributable: 96ms CPU time
Warning::
Third-party scripts: 466ms (100% of total)

Main Thread Breakdown

Script Evaluation 34% Style & Layout 33% Other 21% Script Parsing & Compilation 5% Parse HTML & CSS 4% Rendering 3%
ScriptTotalScriptingParse/CompileParty
https://about.readthedocs.com/_/static/javascript/readthedocs-addons.js205ms115ms17ms3rd
https://about.readthedocs.com/?ref=app.readthedocs.org165ms1ms0ms3rd
Unattributable96ms3ms0ms3rd
A+
Font Loading
19 fonts (1039 KB)
PASS
19 fonts (1039 KB)
Info::
19 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

19

1039 KB total

Render-blocking

0

of 19

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 1.6 s LCP 2.0 shttps://about.readthedocs.com/theme/css/fonts/lato-bold.woff2?cccb897485813c7c256901dbca54ecf2lato-bold.woff2 https://about.readthedocs.com/theme/css/fonts/lato-bold.woff2?cccb897485813c7c256901dbca54ecf2 — 181.4 KiB — 44 ms — font-display: swaphttps://about.readthedocs.com/theme/css/fonts/lato-normal.woff2?bd03a2cc277bbbc338d464e679fe9942lato-normal.woff2 https://about.readthedocs.com/theme/css/fonts/lato-normal.woff2?bd03a2cc277bbbc338d464e679fe9942 — 179.2 KiB — 56 ms — font-display: swaphttps://about.readthedocs.com/theme/css/fonts/lato-heavy.woff2?c9cbbdc3762c340d5d37073a54971487lato-heavy.woff2 https://about.readthedocs.com/theme/css/fonts/lato-heavy.woff2?c9cbbdc3762c340d5d37073a54971487 — 181.0 KiB — 52 ms — font-display: swaphttps://about.readthedocs.com/theme/css/fonts/lato-bold-italic.woff2?0b6bb6725576b072c5d0b02ecdd1900dlato-bold-italic.woff2 https://about.readthedocs.com/theme/css/fonts/lato-bold-italic.woff2?0b6bb6725576b072c5d0b02ecdd1900d — 189.6 KiB — 56 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-1.woff2pro-fa-duotone-900-1.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-1.woff2 — 15.4 KiB — 18 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-brands-400-2.woff2pro-fa-brands-400-2.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-brands-400-2.woff2 — 39.1 KiB — 24 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-brands-400-0.woff2pro-fa-brands-400-0.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-brands-400-0.woff2 — 36.9 KiB — 33 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-0.woff2pro-fa-duotone-900-0.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-0.woff2 — 13.0 KiB — 25 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-28.woff2pro-fa-duotone-900-28.w... https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-28.woff2 — 16.7 KiB — 23 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-5.woff2pro-fa-duotone-900-5.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-5.woff2 — 22.2 KiB — 29 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-14.woff2pro-fa-duotone-900-14.w... https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-14.woff2 — 12.1 KiB — 37 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-24.woff2pro-fa-duotone-900-24.w... https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-24.woff2 — 20.0 KiB — 33 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-13.woff2pro-fa-duotone-900-13.w... https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-13.woff2 — 18.2 KiB — 35 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-6.woff2pro-fa-duotone-900-6.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-6.woff2 — 24.3 KiB — 30 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-3.woff2pro-fa-duotone-900-3.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-3.woff2 — 18.3 KiB — 40 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-16.woff2pro-fa-duotone-900-16.w... https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-16.woff2 — 13.9 KiB — 40 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-2.woff2pro-fa-duotone-900-2.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-2.woff2 — 16.8 KiB — 35 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-7.woff2pro-fa-duotone-900-7.woff2 https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-7.woff2 — 26.4 KiB — 37 ms — font-display: swaphttps://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-19.woff2pro-fa-duotone-900-19.w... https://ka-p.fontawesome.com/releases/v6.7.2/webfonts/pro-fa-duotone-900-19.woff2 — 14.6 KiB — 38 ms — font-display: swap
lato-bold.woff2 woff2 swap
Size 181 KB
Load time 44 ms
Start 746 ms
Risk FOUT — text flashes from fallback to web font
  • Subset this font — over 100 KB suggests Latin Extended or full glyph coverage that most pages don't need
lato-normal.woff2 woff2 swap
Size 179 KB
Load time 56 ms
Start 746 ms
Risk FOUT — text flashes from fallback to web font
  • Subset this font — over 100 KB suggests Latin Extended or full glyph coverage that most pages don't need
lato-heavy.woff2 woff2 swap
Size 181 KB
Load time 52 ms
Start 746 ms
Risk FOUT — text flashes from fallback to web font
  • Subset this font — over 100 KB suggests Latin Extended or full glyph coverage that most pages don't need
lato-bold-italic.woff2 woff2 swap
Size 190 KB
Load time 56 ms
Start 747 ms
Risk FOUT — text flashes from fallback to web font
  • Subset this font — over 100 KB suggests Latin Extended or full glyph coverage that most pages don't need
pro-fa-duotone-900-1.woff2 woff2 swap
Size 15 KB
Load time 18 ms
Start 1.0 s
Risk FOUT — text flashes from fallback to web font
pro-fa-brands-400-2.woff2 woff2 swap
Size 39 KB
Load time 24 ms
Start 1.0 s
Risk FOUT — text flashes from fallback to web font
pro-fa-brands-400-0.woff2 woff2 swap
Size 37 KB
Load time 33 ms
Start 1.0 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-0.woff2 woff2 swap
Size 13 KB
Load time 25 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-28.w... woff2 swap
Size 17 KB
Load time 23 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-5.woff2 woff2 swap
Size 22 KB
Load time 29 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-14.w... woff2 swap
Size 12 KB
Load time 37 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-24.w... woff2 swap
Size 20 KB
Load time 33 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-13.w... woff2 swap
Size 18 KB
Load time 35 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-6.woff2 woff2 swap
Size 24 KB
Load time 30 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-3.woff2 woff2 swap
Size 18 KB
Load time 40 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-16.w... woff2 swap
Size 14 KB
Load time 40 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-2.woff2 woff2 swap
Size 17 KB
Load time 35 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-7.woff2 woff2 swap
Size 26 KB
Load time 37 ms
Start 1.1 s
Risk FOUT — text flashes from fallback to web font
pro-fa-duotone-900-19.w... woff2 swap
Size 15 KB
Load time 38 ms
Start 1.1 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)

Fixing the unchecked items could save ~366 KB and ~732 ms

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
50 requests over 1168ms
INFO
0 ms 500 ms 1.0 s 1.5 s 2.0 s FCP 1.6 s LCP 2.0 shttps://readthedocs.org// https://readthedocs.org/ — 302 — 103 B — 18 ms — h2https://app.readthedocs.org/welcome/app.readthedocs.org/welcome/ https://app.readthedocs.org/welcome/ — 302 — 1.4 KiB — 452 ms — h2https://about.readthedocs.com/?ref=app.readthedocs.orgabout.readthedocs.com/?ref=app.read... https://about.readthedocs.com/?ref=app.readthedocs.org — 200 — 7.3 KiB — 49 ms — h3https://about.readthedocs.com/theme/css/site.cssabout.readthedocs.com/theme/css/sit... https://about.readthedocs.com/theme/css/site.css — 200 — 102.0 KiB — 43 ms — h3https://kit.fontawesome.com/53af4251af.jskit.fontawesome.com/53af4251af.js https://kit.fontawesome.com/53af4251af.js — 200 — 5.7 KiB — 37 ms — h2https://about.readthedocs.com/theme/js/vendor.jsabout.readthedocs.com/theme/js/vend... https://about.readthedocs.com/theme/js/vendor.js — 200 — 116.6 KiB — 45 ms — h3https://about.readthedocs.com/theme/js/site.jsabout.readthedocs.com/theme/js/site... https://about.readthedocs.com/theme/js/site.js — 200 — 2.7 KiB — 39 ms — h3https://about.readthedocs.com/theme/img/logo-wordmark-dark.svgabout.readthedocs.com/theme/img/log... https://about.readthedocs.com/theme/img/logo-wordmark-dark.svg — 200 — 3.7 KiB — 33 ms — h3https://about.readthedocs.com/images/homepage.pngabout.readthedocs.com/images/homepa... https://about.readthedocs.com/images/homepage.png — 200 — 33.1 KiB — 34 ms — h3https://about.readthedocs.com/images/amd-logo.svgabout.readthedocs.com/images/amd-lo... https://about.readthedocs.com/images/amd-logo.svg — 200 — 1.2 KiB — 27 ms — h3https://about.readthedocs.com/images/canonical-logo.svgabout.readthedocs.com/images/canoni... https://about.readthedocs.com/images/canonical-logo.svg — 200 — 3.5 KiB — 27 ms — h3https://ka-p.fontawesome.com/releases/v6.7.2/css/pro.min.css?token=53af4251afka-p.fontawesome.com/releases/v6.7.... https://ka-p.fontawesome.com/releases/v6.7.2/css/pro.min.css?token=53af4251af — 200 — 80.7 KiB — 35 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
100% of JavaScript execution is third-party
First-party Third-party 466ms · 139KB · €5/mo
about.readthedocs.com
Other
Costly
Execution 205ms
Transfer 132 KB
Unused 39%
Monthly Cost €2/mo
about.readthedocs.com
Other
Optional
Execution 165ms
Transfer 7 KB
Monthly Cost €2/mo
Unattributable
Other
Optional
Execution 96ms
Transfer 0 KB
Monthly Cost €1/mo

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100% of JavaScript execution time is spent on third-party scripts. Consider auditing which scripts are essential.

Why this matters

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Source: web.dev

about.readthedocs.com takes 205ms 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

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