Frontend Performance Optimization

Diagnose and fix performance issues in Next.js applications.

Quick Diagnostics

Run Lighthouse CLI

# Performance audit (headless)
npx lighthouse http://localhost:3000 --output=json --output-path=./lighthouse.json --chrome-flags="--headless" --only-categories=performance

# Parse key metrics
cat lighthouse.json | jq '{
  score: .categories.performance.score,
  FCP: .audits["first-contentful-paint"].displayValue,
  LCP: .audits["largest-contentful-paint"].displayValue,
  TBT: .audits["total-blocking-time"].displayValue,
  CLS: .audits["cumulative-layout-shift"].displayValue
}'

# Find slow scripts
cat lighthouse.json | jq '.audits["bootup-time"].details.items | .[0:8]'

# Main thread breakdown
cat lighthouse.json | jq '.audits["mainthread-work-breakdown"].details.items'

Important: Always test production builds (next build && next start), not dev mode. Dev mode has 2-10x overhead from HMR, source maps, and no optimizations.

Bundle Analysis

# Install
bun add -d @next/bundle-analyzer

# Run analysis
ANALYZE=true bun run build

Configure in next.config.js:

import bundleAnalyzer from '@next/bundle-analyzer'

const withBundleAnalyzer = bundleAnalyzer({
  enabled: process.env.ANALYZE === 'true',
})

export default withBundleAnalyzer(nextConfig)

Common Fixes

1. optimizePackageImports

For libraries with many exports (icons, utilities, animation libraries):

// next.config.js
const nextConfig = {
  experimental: {
    optimizePackageImports: [
      'framer-motion',
      'lucide-react',
      '@phosphor-icons/react',
      'lodash',
      'date-fns',
      '@heroicons/react',
    ],
  },
}

This ensures tree-shaking works correctly - only imports you use get bundled.

2. Framer Motion - Variants Pattern

WRONG - Creates N animation controllers:

// Each element has its own animation state - expensive!
{items.map((item, i) => (
  <motion.div
    key={i}
    initial={{ opacity: 0, y: 20 }}
    animate={{ opacity: 1, y: 0 }}
    transition={{ delay: i * 0.05 }}  // Individual delays
  >
    {item}
  </motion.div>
))}

RIGHT - Single controller with staggerChildren:

// Parent controls all children - efficient!
const containerVariants = {
  hidden: {},
  visible: {
    transition: {
      staggerChildren: 0.05,
    },
  },
}

const itemVariants = {
  hidden: { opacity: 0, y: 20 },
  visible: {
    opacity: 1,
    y: 0,
    transition: { type: 'spring' as const, damping: 15 }
  },
}

<motion.div
  variants={containerVariants}
  initial="hidden"
  animate="visible"
>
  {items.map((item, i) => (
    <motion.div key={i} variants={itemVariants}>
      {item}
    </motion.div>
  ))}
</motion.div>

Also avoid:

• filter: blur() in animations - very expensive
• Too many infinite animations (reduce or use CSS)
• Individual transition props on children when using variants

3. Move Heavy Computation Server-Side

Keep these out of client bundles:

• Syntax highlighting: use shiki server-side, not prism-react-renderer
• Markdown parsing: render on server
• Date formatting libraries: consider Intl.DateTimeFormat
• Large data transformations: API routes or server components

4. Image Optimization

// Always use next/image
import Image from 'next/image'

<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority  // For LCP images
  placeholder="blur"  // Reduces CLS
/>

5. Font Optimization

// app/layout.tsx
import { Inter } from 'next/font/google'

const inter = Inter({
  subsets: ['latin'],
  display: 'swap',  // Prevents FOIT
  preload: true,
})

6. External Packages (Server Only)

Prevent server-only packages from being bundled:

// next.config.js
const nextConfig = {
  serverExternalPackages: ['sharp', 'canvas'],
}

Performance Targets

Metric	Good	Needs Improvement	Poor
LCP	≤2.5s	2.5-4s	>4s
FCP	≤1.8s	1.8-3s	>3s
TBT	≤200ms	200-600ms	>600ms
CLS	≤0.1	0.1-0.25	>0.25

Debugging Workflow

1. Run production Lighthouse - Get baseline metrics
2. Check bootup-time audit - Find slow scripts
3. Run bundle analyzer - Identify large chunks
4. Fix largest issues first - Usually 1-2 packages cause most problems
5. Re-test - Verify improvements

Quick Wins Checklist

• Test production build, not dev
• Add optimizePackageImports for icon/utility libraries
• Use Framer Motion variants pattern
• Remove filter: blur() from animations
• Add priority to LCP images
• Use next/font with display: swap
• Move heavy libraries to server components