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https://www.duolingo.com

Performance

· 14 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
72
GRADE
C
FIX
5
REVIEW
2
PASS
5
INFO
2

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

Mobile Desktop
68
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.
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.

1.29 s

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

2.36 s

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

232 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.

3.34 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.

4.28 s

Page Load Progression

2.1 s
4.3 s
6.4 s
8.5 s
10.6 s
12.8 s
14.9 s
17.0 s
Checks
14
5 PASS 2 REVIEW 5 FIX
F
Page Weight Budget
Action
3.3 MB transferred, 140 requests
FIX
3.3 MB transferred, 140 requests
Warning::
Page weighs 13.9 MB (3.3 MB transferred)
Warning::
JavaScript is 1.8 MB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 1.8 MB
Warning::
140 HTTP requests — consider bundling or reducing
Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.
Info::
Estimated 0.69 g CO2 per page load
3.3 MB / 2.4 MB budget
3.3 MB transferred
140 requests
0.69 g CO2 per page load
JavaScript 1.8 MiB 55%
Other 775.1 KiB 23%
Images 403.1 KiB 12%
Fonts 186.2 KiB 6%
CSS 84.1 KiB 3%
HTML 33.1 KiB 1%
Other 15.9 KiB 0%
Other 2.6 KiB 0%
Other 973 B 0%
Other 0 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

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
99% third-party, 0 ms blocking
FIX
99% third-party, 0 ms blocking
Warning::
Third-party code accounts for 99% of page weight (3.3 MiB of 3.3 MiB)
Info::
Third-party blocking time is low (0 ms)
99%
First-party Third-party
F
JS Execution Cost
Action
12777ms total JS execution
FIX
12777ms total JS execution
Warning::
https://d35aaqx5ub95lt.cloudfront.net/js/5314-33cb...: 6986ms CPU time
Warning::
https://www.duolingo.com/: 3681ms CPU time
Warning::
Unattributable: 784ms CPU time
Warning::
https://www.gstatic.com/recaptcha/releases/kUYUkUl...: 492ms CPU time
Warning::
https://d35aaqx5ub95lt.cloudfront.net/js/app-cf122...: 417ms CPU time
Warning::
Third-party scripts: 9096ms (71% of total)

Main Thread Breakdown

Script Evaluation 40% Style & Layout 25% Other 24% Rendering 10%
ScriptTotalScriptingParse/CompileParty
https://d35aaqx5ub95lt.cloudfront.net/js/5314-33cb86aa.js6986ms3930ms3ms3rd
https://www.duolingo.com/3681ms1ms1ms1st
Unattributable784ms8ms0ms3rd
https://www.gstatic.com/recaptcha/releases/kUYUkUlSyqkjTSMaN2w3RaOh/recaptcha__en.js492ms439ms20ms3rd
https://d35aaqx5ub95lt.cloudfront.net/js/app-cf122905.js417ms327ms30ms3rd
https://d35aaqx5ub95lt.cloudfront.net/js/6057-b2849109.js362ms295ms19ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-TX6Z97C54ms43ms10ms3rd
F
JS Bundles
Action
29 scripts, 1.1 MB unused
FIX
29 scripts, 1.1 MB unused
Warning::
https://www.gstatic.com/recaptcha/releases/kUYUkUl...: 179 KB unused (49%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.gstatic.com/recaptcha/releases/kUYUkUl...: 179 KB unused (49%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.gstatic.com/recaptcha/releases/kUYUkUl...: 179 KB unused (49%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://d35aaqx5ub95lt.cloudfront.net/js/app-cf122...: 174 KB unused (55%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://d35aaqx5ub95lt.cloudfront.net/js/6057-b284...: 138 KB unused (58%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 1.1 MB
1094 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.gstatic.com/recaptcha/releases/kUYUkUlSyqkjTSMaN2w3RaOh/recaptcha__en.js0 KB179 KB (49%)✓3rd
https://www.gstatic.com/recaptcha/releases/kUYUkUlSyqkjTSMaN2w3RaOh/recaptcha__en.js363 KB179 KB (49%)✓3rd
https://www.gstatic.com/recaptcha/releases/kUYUkUlSyqkjTSMaN2w3RaOh/recaptcha__en.js363 KB179 KB (49%)✓3rd
https://d35aaqx5ub95lt.cloudfront.net/js/app-cf122905.js316 KB174 KB (55%)✓3rd
https://d35aaqx5ub95lt.cloudfront.net/js/6057-b2849109.js237 KB138 KB (58%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-TX6Z97C166 KB91 KB (55%)✓3rd
https://accounts.google.com/gsi/client94 KB73 KB (79%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-728842606&cx=c&gtm=4e6461h2137 KB60 KB (44%)✓3rd
https://d35aaqx5ub95lt.cloudfront.net/js/5314-33cb86aa.js46 KB21 KB (47%)✓3rd
https://d35aaqx5ub95lt.cloudfront.net/js/8186-1df1f4b0.js4 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

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

D
HTTP Caching
Action
no-store (uncacheable)
FIX
no-store (uncacheable)
Info::
Cache-Control: no-store prevents all caching
Intentional for dynamic content, but increases server load for every page view.
Got: no-store, no-cache, must-revalidate, proxy-revalidate
Info::
No ETag or Last-Modified header
Conditional requests (304 Not Modified) are not possible without validators.
Cache-Control

no-store, no-cache, must-revalidate, proxy-revalidate

DirectiveValueMeaning
no-store—Never cache this response
no-cache—Cache but always revalidate first
must-revalidate—Must recheck with server after expiry
proxy-revalidate—
B
Image Optimization
55 images, 0 KB saveable
REVIEW
55 images, 0 KB saveable
Info::
All images are well-optimized
55 images 403 KB
0 oversized -0 KB
55 legacy format
0 missing dimensions CLS risk
C
Green Hosting
Action
Whether the site is served from green-energy infrastructure
REVIEW

Green Hosting

No green hosting detected

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
4 fonts (186 KB)
PASS
4 fonts (186 KB)
Info::
4 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

4

186 KB total

Render-blocking

0

of 4

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 1.3 s LCP 2.4 shttps://d35aaqx5ub95lt.cloudfront.net/fonts/642e24bb0295f3aee4dedcd8eecd8007.woff2642e24bb0295f3aee4dedcd... https://d35aaqx5ub95lt.cloudfront.net/fonts/642e24bb0295f3aee4dedcd8eecd8007.woff2 — 63.0 KiB — 0 ms — font-display: swaphttps://d35aaqx5ub95lt.cloudfront.net/fonts/a96f1b78ebf295556c0e9500cde6dd64.woff2a96f1b78ebf295556c0e950... https://d35aaqx5ub95lt.cloudfront.net/fonts/a96f1b78ebf295556c0e9500cde6dd64.woff2 — 63.8 KiB — 0 ms — font-display: swaphttps://d35aaqx5ub95lt.cloudfront.net/vendor/68d22534874c554b0c9ebabea379fc60.woff268d22534874c554b0c9ebab... https://d35aaqx5ub95lt.cloudfront.net/vendor/68d22534874c554b0c9ebabea379fc60.woff2 — 25.5 KiB — 0 ms — font-display: swaphttps://fonts.gstatic.com/s/roboto/v48/KFO7CnqEu92Fr1ME7kSn66aGLdTylUAMa3yUBHMdazQ.woff2KFO7CnqEu92Fr1ME7kSn66a... https://fonts.gstatic.com/s/roboto/v48/KFO7CnqEu92Fr1ME7kSn66aGLdTylUAMa3yUBHMdazQ.woff2 — 33.9 KiB — 0 ms — font-display: swap
642e24bb0295f3aee4dedcd... woff2 swap
Size 63 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
a96f1b78ebf295556c0e950... woff2 swap
Size 64 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
68d22534874c554b0c9ebab... woff2 swap
Size 25 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
KFO7CnqEu92Fr1ME7kSn66a... woff2 swap
Size 34 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
Network Waterfall
140 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
71% of JavaScript execution is third-party
First-party Third-party 9096ms · 765KB · €145/mo
d35aaqx5ub95lt.cloudfront.net
Other
Costly
Execution 6986ms
Transfer 46 KB
Unused 47%
Monthly Cost €112/mo
Unattributable
Other
Costly
Execution 784ms
Transfer 0 KB
Monthly Cost €13/mo
www.gstatic.com
Other
Costly
Execution 492ms
Transfer 0 KB
Unused 49%
Monthly Cost €8/mo
d35aaqx5ub95lt.cloudfront.net
Other
Costly
Execution 417ms
Transfer 316 KB
Unused 55%
Monthly Cost €7/mo
d35aaqx5ub95lt.cloudfront.net
Other
Costly
Execution 362ms
Transfer 237 KB
Unused 58%
Monthly Cost €6/mo
Google Tag Manager
Tag Manager
Optional
Execution 54ms
Transfer 166 KB
Unused 55%
Monthly Cost €1/mo

These scripts may cost more than they're worth

  • d35aaqx5ub95lt.cloudfront.net adds 6986ms and costs ~€112/month
  • Unattributable adds 784ms and costs ~€13/month
  • www.gstatic.com adds 492ms and costs ~€8/month
  • d35aaqx5ub95lt.cloudfront.net adds 417ms and costs ~€7/month
  • d35aaqx5ub95lt.cloudfront.net adds 362ms and costs ~€6/month

71% 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

d35aaqx5ub95lt.cloudfront.net takes 6986ms 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

Unattributable takes 784ms 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

www.gstatic.com takes 492ms 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

d35aaqx5ub95lt.cloudfront.net takes 417ms 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

55% of d35aaqx5ub95lt.cloudfront.net'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

d35aaqx5ub95lt.cloudfront.net takes 362ms 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

58% of d35aaqx5ub95lt.cloudfront.net'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

55% of Google Tag Manager'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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