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Performance

· 15 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
84
GRADE
B
FIX
2
REVIEW
4
PASS
7
INFO
2

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

Mobile Desktop
94
Performance Overall performance score (0–100) based on Core Web Vitals and other metrics. 90+ is good.
83
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.

1.11 s

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

1.39 s

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

0 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.

1.11 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.

1.39 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 4 REVIEW 2 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 (999.0 KiB of 999.1 KiB)
Info::
Third-party blocking time is low (0 ms)
100%
First-party Third-party
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.

B
Image Optimization
10 images, 0 KB saveable
REVIEW
10 images, 0 KB saveable
Warning::
https://www.zotero.org/static/images/icons/mastodo... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
10 images 459 KB
0 oversized -0 KB
10 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
1 scripts, 226 KB unused
REVIEW
1 scripts, 226 KB unused
Warning::
https://www.zotero.org/static/js/bs4/web-component...: 226 KB unused (76%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 226 KB
226 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.zotero.org/static/js/bs4/web-components.min.1775251538.js296 KB226 KB (76%)✓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

B
Page Weight Inventory
22 resources · 999 KB · 226 KB savings available
REVIEW
22 resources · 999 KB · 226 KB savings available
Info::
Within 1MB page-weight target
Got: 999 KB
Info::
~226 KB of savings available
Warning::
21 third-party resources (100% of weight)
Info::
2 resources over 200KB
22 resources · 999.1 KiB · 21 third-party Within 1MB page-weight target
Image 46% · 459.6 KiB
JavaScript 30% · 296.2 KiB
Font 19% · 188.4 KiB
CSS 4% · 42.5 KiB
Document 1% · 12.4 KiB

~225.9 KiB of savings available

Remove unused JavaScript: 225.9 KiB
C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

A
Page Weight Budget
999 KB transferred, 22 requests
PASS
999 KB transferred, 22 requests
Info::
Page weighs 2.0 MB (999 KB transferred)
Info::
22 HTTP requests
Info::
Estimated 0.20 g CO2 per page load
999 KB / 2.4 MB budget
999 KB transferred
22 requests
0.20 g CO2 per page load
Images 459.0 KiB 46%
JavaScript 296.2 KiB 30%
Fonts 188.4 KiB 19%
CSS 42.5 KiB 4%
HTML 12.3 KiB 1%
Other 720 B 0%
0 2.4 MB 4.9 MB
Under budget
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
378ms total JS execution
PASS
378ms total JS execution
Info::
https://www.zotero.org/static/js/bs4/web-component...: 212ms CPU time
Info::
https://www.zotero.org/: 94ms CPU time
Info::
Unattributable: 72ms CPU time
Warning::
Third-party scripts: 378ms (100% of total)

Main Thread Breakdown

Other 40% Script Evaluation 21% Style & Layout 18% Rendering 10% Script Parsing & Compilation 9%
ScriptTotalScriptingParse/CompileParty
https://www.zotero.org/static/js/bs4/web-components.min.1775251538.js212ms77ms32ms3rd
https://www.zotero.org/94ms2ms1ms3rd
Unattributable72ms3ms0ms3rd
A+
Font Loading
6 fonts (188 KB)
PASS
6 fonts (188 KB)
Info::
6 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

6

188 KB total

Render-blocking

0

of 6

Dominant font-display

swap

Most common across fonts

Font loading timeline
Transfer FOIT (block) FOUT (swap)
0 ms 500 ms 1.0 s FCP 1.1 s LCP 1.4 shttps://www.zotero.org/static/fonts/36AC02_5_0.woff236AC02_5_0.woff2 https://www.zotero.org/static/fonts/36AC02_5_0.woff2 — 29.8 KiB — 20 ms — font-display: swaphttps://www.zotero.org/static/fonts/36AC02_4_0.woff236AC02_4_0.woff2 https://www.zotero.org/static/fonts/36AC02_4_0.woff2 — 32.6 KiB — 20 ms — font-display: swaphttps://www.zotero.org/static/fonts/36AC02_3_0.woff236AC02_3_0.woff2 https://www.zotero.org/static/fonts/36AC02_3_0.woff2 — 33.6 KiB — 13 ms — font-display: swaphttps://www.zotero.org/static/fonts/36AC02_2_0.woff236AC02_2_0.woff2 https://www.zotero.org/static/fonts/36AC02_2_0.woff2 — 31.1 KiB — 14 ms — font-display: swaphttps://www.zotero.org/static/fonts/36AC02_1_0.woff236AC02_1_0.woff2 https://www.zotero.org/static/fonts/36AC02_1_0.woff2 — 30.1 KiB — 16 ms — font-display: swaphttps://www.zotero.org/static/fonts/36AC02_0_0.woff236AC02_0_0.woff2 https://www.zotero.org/static/fonts/36AC02_0_0.woff2 — 31.2 KiB — 18 ms — font-display: swap
36AC02_5_0.woff2 woff2 swap
Size 30 KB
Load time 20 ms
Start 319 ms
Risk FOUT — text flashes from fallback to web font
36AC02_4_0.woff2 woff2 swap
Size 33 KB
Load time 20 ms
Start 320 ms
Risk FOUT — text flashes from fallback to web font
36AC02_3_0.woff2 woff2 swap
Size 34 KB
Load time 13 ms
Start 338 ms
Risk FOUT — text flashes from fallback to web font
36AC02_2_0.woff2 woff2 swap
Size 31 KB
Load time 14 ms
Start 370 ms
Risk FOUT — text flashes from fallback to web font
36AC02_1_0.woff2 woff2 swap
Size 30 KB
Load time 16 ms
Start 371 ms
Risk FOUT — text flashes from fallback to web font
36AC02_0_0.woff2 woff2 swap
Size 31 KB
Load time 18 ms
Start 371 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
22 requests over 407ms
INFO
0 ms 500 ms 1.0 s FCP 1.1 s LCP 1.4 shttps://zotero.org// https://zotero.org/ — 301 — 159 B — 47 ms — h2https://www.zotero.org/www.zotero.org/ https://www.zotero.org/ — 200 — 12.3 KiB — 27 ms — h2https://www.zotero.org/static/css/bs4/web-components.min.1775251542.csswww.zotero.org/static/css/bs4/web-c... https://www.zotero.org/static/css/bs4/web-components.min.1775251542.css — 200 — 42.1 KiB — 29 ms — h2https://www.zotero.org/static/js/bs4/web-components.min.1775251538.jswww.zotero.org/static/js/bs4/web-co... https://www.zotero.org/static/js/bs4/web-components.min.1775251538.js — 200 — 296.2 KiB — 62 ms — h2https://www.zotero.org/static/images/home/screenshot-7.0.pngwww.zotero.org/static/images/home/s... https://www.zotero.org/static/images/home/screenshot-7.0.png — 200 — 431.0 KiB — 47 ms — h2https://www.zotero.org/static/images/home/static-collect.svgwww.zotero.org/static/images/home/s... https://www.zotero.org/static/images/home/static-collect.svg — 200 — 6.8 KiB — 18 ms — h2https://www.zotero.org/static/images/home/organize.svgwww.zotero.org/static/images/home/o... https://www.zotero.org/static/images/home/organize.svg — 200 — 3.5 KiB — 22 ms — h2https://hello.myfonts.net/count/36ac02hello.myfonts.net/count/36ac02 https://hello.myfonts.net/count/36ac02 — 200 — 372 B — 40 ms — h2https://www.zotero.org/static/images/home/cite.svgwww.zotero.org/static/images/home/c... https://www.zotero.org/static/images/home/cite.svg — 200 — 1.6 KiB — 11 ms — h2https://www.zotero.org/static/images/home/sync-xl.svgwww.zotero.org/static/images/home/s... https://www.zotero.org/static/images/home/sync-xl.svg — 200 — 1.5 KiB — 12 ms — h2https://www.zotero.org/static/images/bs4theme/zotero-logo.1775160902.svgwww.zotero.org/static/images/bs4the... https://www.zotero.org/static/images/bs4theme/zotero-logo.1775160902.svg — 200 — 1.5 KiB — 11 ms — h2https://www.zotero.org/static/images/home/collaborate.svgwww.zotero.org/static/images/home/c... https://www.zotero.org/static/images/home/collaborate.svg — 200 — 7.7 KiB — 11 ms — h2https://www.zotero.org/static/images/home/privacy.svgwww.zotero.org/static/images/home/p... https://www.zotero.org/static/images/home/privacy.svg — 200 — 742 B — 10 ms — h2https://www.zotero.org/static/images/icons/mastodon-logo-white.svgwww.zotero.org/static/images/icons/... https://www.zotero.org/static/images/icons/mastodon-logo-white.svg — 200 — 1.3 KiB — 10 ms — h2https://www.zotero.org/static/images/home/jumbotron.svgwww.zotero.org/static/images/home/j... https://www.zotero.org/static/images/home/jumbotron.svg — 200 — 3.6 KiB — 10 ms — h2https://www.zotero.org/static/fonts/36AC02_5_0.woff2www.zotero.org/static/fonts/36AC02_... https://www.zotero.org/static/fonts/36AC02_5_0.woff2 — 200 — 29.8 KiB — 20 ms — h2https://www.zotero.org/static/fonts/36AC02_4_0.woff2www.zotero.org/static/fonts/36AC02_... https://www.zotero.org/static/fonts/36AC02_4_0.woff2 — 200 — 32.6 KiB — 20 ms — h2https://www.zotero.org/static/fonts/36AC02_3_0.woff2www.zotero.org/static/fonts/36AC02_... https://www.zotero.org/static/fonts/36AC02_3_0.woff2 — 200 — 33.6 KiB — 13 ms — h2https://www.zotero.org/static/fonts/36AC02_2_0.woff2www.zotero.org/static/fonts/36AC02_... https://www.zotero.org/static/fonts/36AC02_2_0.woff2 — 200 — 31.1 KiB — 14 ms — h2https://www.zotero.org/static/fonts/36AC02_1_0.woff2www.zotero.org/static/fonts/36AC02_... https://www.zotero.org/static/fonts/36AC02_1_0.woff2 — 200 — 30.1 KiB — 16 ms — h2https://www.zotero.org/static/fonts/36AC02_0_0.woff2www.zotero.org/static/fonts/36AC02_... https://www.zotero.org/static/fonts/36AC02_0_0.woff2 — 200 — 31.2 KiB — 18 ms — h2https://www.zotero.org/favicon.icowww.zotero.org/favicon.ico https://www.zotero.org/favicon.ico — 200 — 561 B — 10 ms — h2
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 378ms · 308KB · €4/mo
www.zotero.org
Other
Costly
Execution 212ms
Transfer 296 KB
Unused 76%
Monthly Cost €2/mo
www.zotero.org
Other
Optional
Execution 94ms
Transfer 12 KB
Monthly Cost €1/mo
Unattributable
Other
Optional
Execution 72ms
Transfer 0 KB
Monthly Cost €1/mo

These scripts may cost more than they're worth

  • www.zotero.org adds 212ms and costs ~€2/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

www.zotero.org takes 212ms 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

76% of www.zotero.org'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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