Web Performance Optimization

Performance

Optimize web performance: bundle size, images, caching, lazy loading, and overall page speed. Use when site is slow, reducing bundle size, fixing layout shifts, improving Time to Interactive, or optimizing for Lighthouse scores. Triggers on: web performance, bundle size, page speed, slow site, lazy loading. Do NOT use for Core Web Vitals-specific fixes (use core-web-vitals), running Lighthouse audits (use perf-lighthouse), or Astro-specific optimization (use perf-astro).

npx @tech-leads-club/agent-skills install --skill perf-web-optimization

What Web Performance Optimization does

Optimize web performance: bundle size, images, caching, lazy loading, and overall page speed.

When to use it

  • site is slow
  • reducing bundle size
  • fixing layout shifts
  • improving Time to Interactive
  • optimizing for Lighthouse scores. Triggers on: web performance
  • bundle size
  • page speed
  • slow site
  • lazy loading

When not to use it

  • Core Web Vitals-specific fixes (use core-web-vitals)
  • running Lighthouse audits (use perf-lighthouse)
  • Astro-specific optimization (use perf-astro)

Web Performance Optimization

Systematic approach: Measure → Identify → Prioritize → Implement → Verify.

Target Metrics

MetricGoodNeeds WorkPoor
LCP< 2.5s2.5-4s> 4s
INP< 200ms200-500ms> 500ms
CLS< 0.10.1-0.25> 0.25
TTFB< 800ms800ms-1.8s> 1.8s

Quick Wins

1. Images (usually biggest impact on LCP)

<!-- Hero/LCP image: eager + high priority -->
<img src="/hero.webp" alt="Hero" width="1200" height="600" loading="eager" fetchpriority="high" decoding="async" />

<!-- Below fold: lazy load -->
<img src="/product.webp" alt="Product" width="400" height="300" loading="lazy" decoding="async" />

Always set width and height to prevent CLS.

2. Fonts (common LCP/CLS culprit)

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

<!-- Non-blocking font load -->
<link
  rel="stylesheet"
  href="https://fonts.googleapis.com/css2?family=Inter&display=swap"
  media="print"
  onload="this.media='all'"
/>

3. Third-party Scripts (common INP killer)

<!-- Defer to user interaction -->
<script>
  function loadThirdParty() {
    // Load analytics, chat widgets, etc.
  }
  ;['scroll', 'click', 'touchstart'].forEach((e) => addEventListener(e, loadThirdParty, { once: true, passive: true }))
  setTimeout(loadThirdParty, 5000)
</script>

4. Critical CSS

Inline critical CSS in <head>, defer the rest:

<style>
  /* critical styles */
</style>
<link rel="preload" href="/styles.css" as="style" onload="this.rel='stylesheet'" />

Bundle Analysis

# Webpack
npx webpack-bundle-analyzer dist/stats.json

# Vite
npx vite-bundle-visualizer

# Check package size before installing
npx bundlephobia <package-name>

Common heavy packages to replace:

  • moment (67KB) → date-fns (12KB) or dayjs (2KB)
  • lodash (72KB) → cherry-pick imports or native methods

Code Splitting Patterns

// React lazy
const Chart = lazy(() => import('./Chart'))

// Next.js dynamic
const Admin = dynamic(() => import('./Admin'), { ssr: false })

// Vite/Rollup manual chunks
build: {
  rollupOptions: {
    output: {
      manualChunks: {
        vendor: ['react', 'react-dom']
      }
    }
  }
}

Caching Headers

# Static assets (immutable hash in filename)
Cache-Control: public, max-age=31536000, immutable

# HTML (revalidate)
Cache-Control: no-cache

# API responses
Cache-Control: private, max-age=0, must-revalidate

Measurement

For running audits, reading reports, and setting budgets, use the perf-lighthouse skill.

Checklist

Images

  • Modern formats (WebP/AVIF)
  • Responsive srcset
  • width/height attributes
  • loading="lazy" below fold
  • fetchpriority="high" on LCP image

JavaScript

  • Bundle < 200KB gzipped
  • Code splitting by route
  • Third-party scripts deferred
  • No unused dependencies

CSS

  • Critical CSS inlined
  • Non-critical CSS deferred
  • No unused CSS

Fonts

  • font-display: swap
  • Preconnect to font origin
  • Subset if possible

Detailed Examples

For in-depth optimization patterns, see:

  • references/core-web-vitals.md - Fixing LCP, CLS, INP issues
  • references/bundle-optimization.md - Reducing JS bundle size
  • references/image-optimization.md - Image formats, responsive images, sharp scripts

Related Performance skills

  • Astro Performance Playbook

    Astro-specific performance optimizations for 95+ Lighthouse scores. Covers critical CSS inlining, compression, font loading, and LCP optimization.

  • Core Web Vitals optimization

    Optimize Core Web Vitals (LCP, INP, CLS) for better page experience and search ranking.

  • Lighthouse Audits

    Run Lighthouse audits locally via CLI or Node API, parse and interpret reports, and set performance budgets.