Performance and Web Vitals

Run a Lighthouse Audit

# CLI audit — outputs JSON and HTML report
npx lighthouse https://example.com --output html --output-path ./lighthouse-report.html

# Headless, useful in CI
npx lighthouse https://example.com --chrome-flags="--headless" --output json --output-path ./report.json

# Audit specific categories only
npx lighthouse https://example.com --only-categories=performance,accessibility,seo

Or open Chrome DevTools → Lighthouse tab → Analyse page load.

Target scores:

Category	Target
Performance	≥ 90
Accessibility	100
Best Practices	≥ 95
SEO	≥ 95

Core Web Vitals

LCP — Largest Contentful Paint

How fast does the main content appear?

Target: ≤ 2.5s

LCP measures when the largest visible element (hero image, heading, video poster) renders. It is the user's perception of "did the page load?"

Common causes and fixes:

Cause	Fix
Unoptimised hero image	Use WebP/AVIF, correct size, `fetchpriority="high"`
Image not preloaded	`<link rel="preload" as="image" href="hero.webp">`
Render-blocking CSS/JS	Defer non-critical JS, inline critical CSS
Slow server response	CDN, caching headers, edge delivery
Web font blocking render	`font-display: swap` or `optional`

<!-- Preload LCP image -->
<link rel="preload" as="image" href="hero.webp" fetchpriority="high">

<!-- LCP image: no lazy loading -->
<img src="hero.webp" alt="..." fetchpriority="high" width="1200" height="600">

Never use loading="lazy" on the LCP image — it delays the most important render.

CLS — Cumulative Layout Shift

Does content jump around while loading?

Target: ≤ 0.1

CLS measures unexpected layout shifts — content moving after it has rendered. Caused by images without dimensions, late-loading ads, fonts swapping, or dynamic content injected above existing content.

Common causes and fixes:

Cause	Fix
Images without width/height	Always set `width` and `height` on `<img>`
Web font swap	Use `font-display: optional` or preload fonts
Dynamic content above fold	Reserve space with `min-height` on containers
Late-loading ads or embeds	Reserve fixed dimensions for ad slots
Animations that shift layout	Animate `transform` only, never `top/left/width/height`

<!-- Always include dimensions -->
<img src="product.jpg" width="400" height="300" alt="...">

/* Reserve space for dynamic content */
.ad-slot { min-height: 250px; }

/* Animate transform, not layout properties */
.slide-in { transform: translateY(0); transition: transform 300ms; }

INP — Interaction to Next Paint

How quickly does the page respond to user input?

Target: ≤ 200ms

INP measures the delay between a user interaction (click, tap, keyboard) and the next visual update. High INP makes the UI feel sluggish or frozen.

Common causes and fixes:

Cause	Fix
Heavy JS on main thread	Break into smaller tasks, use `requestIdleCallback`
Large event handlers	Debounce/throttle scroll and resize handlers
Synchronous DOM updates	Batch DOM writes with `requestAnimationFrame`
Third-party scripts blocking	Load third-party scripts with `async` or `defer`
React re-renders	Memoize with `useMemo`, `useCallback`, `React.memo`

Images

Images are the single biggest performance lever on most pages.

<!-- Modern formats with fallback -->
<picture>
  <source srcset="image.avif" type="image/avif">
  <source srcset="image.webp" type="image/webp">
  <img src="image.jpg" width="800" height="600" alt="..." loading="lazy">
</picture>

<!-- Responsive images -->
<img
  srcset="image-400.webp 400w, image-800.webp 800w, image-1200.webp 1200w"
  sizes="(max-width: 600px) 100vw, 50vw"
  src="image-800.webp"
  alt="..."
  width="800"
  height="600"
  loading="lazy"
>

Rules:

Always set width and height — prevents CLS
Use loading="lazy" below the fold, never on LCP image
Serve WebP or AVIF — typically 30–50% smaller than JPEG
Size images to their display size — do not serve 2000px image for a 400px slot
Use a CDN with automatic format conversion where possible

Fonts

Web fonts block rendering if not handled correctly.

<!-- Preconnect to font origin -->
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>

<!-- Preload critical font file -->
<link rel="preload" href="/fonts/brand.woff2" as="font" type="font/woff2" crossorigin>

@font-face {
  font-family: 'Brand';
  src: url('/fonts/brand.woff2') format('woff2');
  font-display: swap;     /* show fallback immediately, swap when loaded */
  /* font-display: optional; — never swap, use fallback if not cached */
}

font-display: swap — good for headings, acceptable CLS
font-display: optional — zero CLS, font only used if cached (best for body text)
Subset fonts to the characters actually used — reduces file size by 60–80%

JavaScript

<!-- Defer non-critical scripts -->
<script src="analytics.js" defer></script>
<script src="chat-widget.js" async></script>

<!-- Module scripts are deferred by default -->
<script type="module" src="app.js"></script>

defer: executes after HTML parsed, in order — use for most scripts
async: executes as soon as downloaded, out of order — use for independent scripts (analytics)
Never block the main thread with synchronous <script> in <head>

Lighthouse CI (automated audits)

Run Lighthouse in CI to catch regressions before deployment.

# Install
npm install -g @lhci/cli

# Run
lhci autorun --upload.target=temporary-public-storage

# .lighthouserc.json
{
  "ci": {
    "assert": {
      "assertions": {
        "categories:performance": ["warn", { "minScore": 0.9 }],
        "categories:accessibility": ["error", { "minScore": 1.0 }],
        "categories:seo": ["warn", { "minScore": 0.95 }]
      }
    }
  }
}

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Drop this on your repo README

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Performance and Web Vitals

Run a Lighthouse Audit

Core Web Vitals

LCP — Largest Contentful Paint

CLS — Cumulative Layout Shift

INP — Interaction to Next Paint

Images

Fonts

JavaScript

Lighthouse CI (automated audits)

Review Checklist

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