Skip to content
https://kotlinlang.org

Performance

· 14 checks — Lighthouse + waterfall + sustainability rolled into one auditable list.
SCORE
72
GRADE
C
FIX
4
REVIEW
3
PASS
5
INFO
2

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

Mobile Desktop
62
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.

1.69 s

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

2.10 s

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

318 ms

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

0.034

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

3.88 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.

7.35 s

Page Load Progression

550 ms
1.1 s
1.6 s
2.2 s
2.8 s
3.3 s
3.9 s
4.4 s
Checks
14
5 PASS 3 REVIEW 4 FIX
F
Page Weight Budget
Action
3.9 MB transferred, 155 requests
FIX
3.9 MB transferred, 155 requests
Warning::
Page weighs 9.5 MB (3.9 MB transferred)
Warning::
JavaScript is 2.3 MB — consider code splitting or lazy loading
Large JavaScript bundles delay interactivity. Split code by route or defer non-critical scripts.
Got: 2.3 MB
Warning::
Images are 1.1 MB — compress or use modern formats
Convert images to WebP/AVIF and resize to display dimensions to reduce transfer size.
Got: 1.1 MB
Info::
Fonts are 307 KB — consider subsetting or using system fonts
Subset fonts to include only used characters, or switch to system font stacks.
Got: 307 KB
Warning::
155 HTTP requests — consider bundling or reducing
Each request adds latency. Bundle small files, use sprites, or eliminate unnecessary requests.
Info::
Estimated 0.81 g CO2 per page load
3.9 MB / 2.4 MB budget
3.9 MB transferred
155 requests
0.81 g CO2 per page load
JavaScript 2.3 MiB 59%
Images 1.1 MiB 28%
Fonts 306.6 KiB 8%
HTML 99.4 KiB 3%
CSS 51.0 KiB 1%
Other 35.9 KiB 1%
Other 11.0 KiB 0%
Other 2.9 KiB 0%
Other 2.2 KiB 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

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

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

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
JS Execution Cost
Action
2120ms total JS execution
FIX
2120ms total JS execution
Warning::
Unattributable: 279ms CPU time
Info::
https://www.googletagmanager.com/gtm.js?id=GTM-5P9...: 238ms CPU time
Info::
https://www.googletagmanager.com/gtag/js?id=AW-107...: 217ms CPU time
Info::
https://cdn.cookiehub.eu/c2/48464916.js: 202ms CPU time
Info::
https://kotlinlang.org/: 179ms CPU time
Warning::
Third-party scripts: 1545ms (73% of total)

Main Thread Breakdown

Script Evaluation 69% Other 11% Script Parsing & Compilation 11% Style & Layout 5%
ScriptTotalScriptingParse/CompileParty
Unattributable279ms58ms0ms3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5P98238ms204ms27ms3rd
https://www.googletagmanager.com/gtag/js?id=AW-1071903267&cx=c&gtm=4e6460217ms199ms14ms3rd
https://cdn.cookiehub.eu/c2/48464916.js202ms158ms6ms3rd
https://kotlinlang.org/179ms36ms0ms1st
https://cdn.optimizely.com/js/26633200186.js165ms138ms12ms3rd
https://kotlinlang.org/_next/static/chunks/0883b4f9.f93badb8ae74b3c8.js153ms117ms15ms1st
https://www.googletagmanager.com/gtag/js?id=G-9J976DJZ68&cx=c&gtm=4e6460119ms90ms27ms3rd
https://scripts.clarity.ms/0.8.59/clarity.js111ms93ms3ms3rd
https://kotlinlang.org/_next/static/chunks/framework-945b357d4a851f4b.js102ms87ms7ms1st

Showing top 10 of 15 scripts

F
JS Bundles
Action
57 scripts, 664 KB unused
FIX
57 scripts, 664 KB unused
Warning::
https://www.googletagmanager.com/gtag/js?id=AW-963...: 105 KB unused (72%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtm.js?id=GTM-NL2...: 73 KB unused (44%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://kotlinlang.org/_next/static/chunks/0883b4f...: 67 KB unused (47%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://analytics.tiktok.com/i18n/pixel/static/mai...: 63 KB unused (60%)
Consider code splitting or tree shaking to reduce unused code.
Warning::
https://www.googletagmanager.com/gtag/js?id=G-9J97...: 61 KB unused (35%)
Consider code splitting or tree shaking to reduce unused code.
Info::
Total unused JavaScript: 664 KB
664 KB unused JavaScript

Downloaded but never executed on this page.

ScriptSizeUnusedMinifiedParty
https://www.googletagmanager.com/gtag/js?id=AW-963127096&cx=c&gtm=4e6460146 KB105 KB (72%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-NL29B334&gtm=4e6460167 KB73 KB (44%)✓3rd
https://kotlinlang.org/_next/static/chunks/0883b4f9.f93badb8ae74b3c8.js144 KB67 KB (47%)✓1st
https://analytics.tiktok.com/i18n/pixel/static/main.MWE0ZWQ3ZWQwMA.js106 KB63 KB (60%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-9J976DJZ68&cx=c&gtm=4e6460174 KB61 KB (35%)✓3rd
https://www.googletagmanager.com/gtag/js?id=G-M8TDRLXFQH&cx=c&gtm=4e6460143 KB59 KB (42%)✓3rd
https://www.googletagmanager.com/gtag/js?id=AW-1071903267&cx=c&gtm=4e6460146 KB54 KB (37%)✓3rd
https://www.googletagmanager.com/gtm.js?id=GTM-5P98204 KB52 KB (26%)✓3rd
https://cdn.optimizely.com/js/26633200186.js104 KB37 KB (35%)✓3rd
https://connect.facebook.net/en_US/fbevents.js95 KB34 KB (36%)✓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 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.
Info::
ETag present — enables conditional (304) responses
Info::
Last-Modified header present

No Cache-Control header

Adding a Cache-Control header can significantly improve repeat-visit performance.

B
Third-Party Impact
50% third-party, 0 ms blocking
REVIEW
50% third-party, 0 ms blocking
Warning::
Third-party code accounts for 50% of page weight (1.9 MiB of 3.9 MiB)
Info::
Third-party blocking time is low (0 ms)
50%
50%
First-party Third-party
C
Image Optimization
Action
44 images, 0 KB saveable
REVIEW
44 images, 0 KB saveable
Warning::
https://img.youtube.com/vi/JGvk4M0Rfxo/maxresdefau... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
Warning::
https://kotlinlang.org/_next/static/chunks/images/... is missing width/height — may cause layout shift
Set explicit width and height to prevent CLS.
44 images 1.1 MB
0 oversized -0 KB
28 legacy format
8 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

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
6 fonts (307 KB)
PASS
6 fonts (307 KB)
Info::
6 font(s) use font-display: swap (FOUT risk but functional)

Web fonts

6

307 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 1.5 s 2.0 s FCP 1.7 s LCP 2.1 shttps://resources.jetbrains.com/storage/jetbrains-sans/google-fonts/v1.309/webfonts/JetBrainsSans-Regular.woff2JetBrainsSans-Regular.w... https://resources.jetbrains.com/storage/jetbrains-sans/google-fonts/v1.309/webfonts/JetBrainsSans-Regular.woff2 — 38.1 KiB — 0 ms — font-display: swaphttps://resources.jetbrains.com/storage/jetbrains-sans/google-fonts/v1.309/webfonts/JetBrainsSans-SemiBold.woff2JetBrainsSans-SemiBold.... https://resources.jetbrains.com/storage/jetbrains-sans/google-fonts/v1.309/webfonts/JetBrainsSans-SemiBold.woff2 — 39.2 KiB — 0 ms — font-display: swaphttps://resources.jetbrains.com/storage/jetbrains-sans/google-fonts/v1.309/variable/JetBrainsSans[wght].woff2JetBrainsSans[wght].woff2 https://resources.jetbrains.com/storage/jetbrains-sans/google-fonts/v1.309/variable/JetBrainsSans[wght].woff2 — 59.2 KiB — 0 ms — font-display: swaphttps://resources.jetbrains.com/storage/jetbrains-mono/v2.304/webfonts/JetBrainsMono-Bold.woff2JetBrainsMono-Bold.woff2 https://resources.jetbrains.com/storage/jetbrains-mono/v2.304/webfonts/JetBrainsMono-Bold.woff2 — 92.9 KiB — 0 ms — font-display: swaphttps://fonts.gstatic.com/l/font?kit=tDbY2o-flEEny0FZhsfKu5WU4zr3E_BX0PnT8RD8yKxTOlOTk6OThhvA&skey=48ad01c60053c2ae&v=v24font https://fonts.gstatic.com/l/font?kit=tDbY2o-flEEny0FZhsfKu5WU4zr3E_BX0PnT8RD8yKxTOlOTk6OThhvA&skey=48ad01c60053c2ae&v=v24 — 25.8 KiB — 0 ms — font-display: swaphttps://fonts.gstatic.com/l/font?kit=tDbY2o-flEEny0FZhsfKu5WU4zr3E_BX0PnT8RD8yKxjPg&skey=48ad01c60053c2ae&v=v24font https://fonts.gstatic.com/l/font?kit=tDbY2o-flEEny0FZhsfKu5WU4zr3E_BX0PnT8RD8yKxjPg&skey=48ad01c60053c2ae&v=v24 — 51.3 KiB — 0 ms — font-display: swap
JetBrainsSans-Regular.w... woff2 swap
Size 38 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
JetBrainsSans-SemiBold.... woff2 swap
Size 39 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
JetBrainsSans[wght].woff2 woff2 swap
Size 59 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
JetBrainsMono-Bold.woff2 woff2 swap
Size 93 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
font unknown swap
Size 26 KB
Load time 0 ms
Start 0 ms
Risk FOUT — text flashes from fallback to web font
font unknown swap
Size 51 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)

Fixing the unchecked items could save ~23 KB and ~46 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
Network Waterfall
155 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
73% of JavaScript execution is third-party
First-party Third-party 1545ms · 1109KB · €27/mo
Unattributable
Other
Costly
Execution 279ms
Transfer 0 KB
Monthly Cost €5/mo
Google Tag Manager
Tag Manager
Costly
Execution 238ms
Transfer 204 KB
Unused 26%
Monthly Cost €4/mo
Google Tag Manager
Tag Manager
Costly
Execution 217ms
Transfer 146 KB
Unused 37%
Monthly Cost €4/mo
cdn.cookiehub.eu
Other
Costly
Execution 202ms
Transfer 38 KB
Monthly Cost €4/mo
Optimizely
A/B Testing
Optional
Execution 165ms
Transfer 104 KB
Unused 35%
Monthly Cost €3/mo
Google Tag Manager
Tag Manager
Optional
Execution 119ms
Transfer 174 KB
Unused 35%
Monthly Cost €2/mo
Microsoft Clarity
Session Replay
Optional
Execution 111ms
Transfer 26 KB
Monthly Cost €2/mo
Google Tag Manager
Tag Manager
Optional
Execution 99ms
Transfer 167 KB
Unused 44%
Monthly Cost €2/mo
analytics.tiktok.com
Other
Optional
Execution 59ms
Transfer 106 KB
Unused 60%
Monthly Cost €1/mo
Google Tag Manager
Tag Manager
Optional
Execution 56ms
Transfer 143 KB
Unused 42%
Monthly Cost €1/mo

These scripts may cost more than they're worth

  • Unattributable adds 279ms and costs ~€5/month
  • Google Tag Manager adds 238ms and costs ~€4/month
  • Google Tag Manager adds 217ms and costs ~€4/month
  • cdn.cookiehub.eu adds 202ms and costs ~€4/month

73% 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 279ms 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

Google Tag Manager takes 238ms 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

Google Tag Manager takes 217ms 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

cdn.cookiehub.eu takes 202ms 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

60% of analytics.tiktok.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.

Send Feedback