react-performance-optimization

name	react-performance-optimization
description	React performance optimization patterns using memoization, code splitting, and efficient rendering strategies. Use when optimizing slow React applications, reducing bundle size, or improving user experience with large datasets.

React Performance Optimization

Expert guidance for optimizing React application performance through memoization, code splitting, virtualization, and efficient rendering strategies.

When to Use This Skill

• Optimizing slow-rendering React components
• Reducing bundle size for faster initial load times
• Improving responsiveness for large lists or data tables
• Preventing unnecessary re-renders in complex component trees
• Optimizing state management to reduce render cascades
• Improving perceived performance with code splitting
• Debugging performance issues with React DevTools Profiler

Core Concepts

React Rendering Optimization

React re-renders components when props or state change. Unnecessary re-renders waste CPU cycles and degrade user experience. Key optimization techniques:

• Memoization: Cache component renders and computed values
• Code splitting: Load code on demand for faster initial loads
• Virtualization: Render only visible list items
• State optimization: Structure state to minimize render cascades

When to Optimize

1. Profile first: Use React DevTools Profiler to identify actual bottlenecks
2. Measure impact: Verify optimization improves performance
3. Avoid premature optimization: Don't optimize fast components

Quick Reference

Load detailed patterns and examples as needed:

Topic	Reference File
React.memo, useMemo, useCallback patterns	.claude/skills/react-performance-optimization/references/memoization.md
Code splitting with lazy/Suspense, bundle optimization	.claude/skills/react-performance-optimization/references/code-splitting.md
Virtualization for large lists (react-window)	.claude/skills/react-performance-optimization/references/virtualization.md
State management strategies, context splitting	.claude/skills/react-performance-optimization/references/state-management.md
useTransition, useDeferredValue (React 18+)	.claude/skills/react-performance-optimization/references/concurrent-features.md
React DevTools Profiler, performance monitoring	.claude/skills/react-performance-optimization/references/profiling-debugging.md
Common pitfalls and anti-patterns	.claude/skills/react-performance-optimization/references/common-pitfalls.md

Optimization Workflow

1. Identify Bottlenecks

# Open React DevTools Profiler
# Record interaction → Analyze flame graph → Find slow components

Look for:

• Components with yellow/red bars (slow renders)
• Unnecessary renders (same props/state)
• Expensive computations on every render

2. Apply Targeted Optimizations

For unnecessary re-renders:

• Wrap component with React.memo
• Use useCallback for stable function references
• Check for inline objects/arrays in props

For expensive computations:

• Use useMemo to cache results
• Move calculations outside render when possible

For large lists:

• Implement virtualization with react-window
• Ensure proper unique keys (not index)

For slow initial load:

• Add code splitting with React.lazy
• Analyze bundle size with webpack-bundle-analyzer
• Use dynamic imports for heavy dependencies

3. Verify Improvements

# Record new Profiler session
# Compare before/after metrics
# Ensure optimization actually helped

Common Patterns

Memoize Expensive Components

import { memo } from 'react'

const ExpensiveList = memo(({ items, onItemClick }) => {
  return items.map(item => (
    <Item key={item.id} data={item} onClick={onItemClick} />
  ))
})

Cache Computed Values

import { useMemo } from 'react'

function DataTable({ items, filters }) {
  const filteredItems = useMemo(() => {
    return items.filter(item => filters.includes(item.category))
  }, [items, filters])

  return <Table data={filteredItems} />
}

Stable Function References

import { useCallback } from 'react'

function Parent() {
  const handleClick = useCallback(id => {
    console.log('Clicked:', id)
  }, [])

  return <MemoizedChild onClick={handleClick} />
}

Code Split Routes

import { lazy, Suspense } from 'react'

const Dashboard = lazy(() => import('./Dashboard'))
const Reports = lazy(() => import('./Reports'))

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <Routes>
        <Route path='/' element={<Dashboard />} />
        <Route path='/reports' element={<Reports />} />
      </Routes>
    </Suspense>
  )
}

Virtualize Large Lists

import { FixedSizeList } from 'react-window'

const Row = ({ index, style, data }) => (
  <div style={style}>{data[index].name}</div>
)

function VirtualList({ items }) {
  return (
    <FixedSizeList
      height={600}
      itemCount={items.length}
      itemSize={80}
      width='100%'
      itemData={items}
    >
      {Row}
    </FixedSizeList>
  )
}

Common Mistakes

1. Over-memoization: Don't memoize simple, fast components (adds overhead)
2. Inline objects/arrays: New references break memoization (config={{ theme: 'dark' }})
3. Missing dependencies: Stale closures in useCallback/useMemo
4. Index as key: Breaks reconciliation when list order changes
5. Single large context: Causes widespread re-renders on any update
6. No profiling: Optimizing without measuring wastes time

Performance Checklist

Before optimizing:

• Profile with React DevTools to identify bottlenecks
• Measure baseline performance metrics

Optimization targets:

• Memoize expensive components with stable props
• Cache computed values with useMemo (if actually expensive)
• Use useCallback for functions passed to memoized children
• Implement code splitting for routes and heavy components
• Virtualize lists with >100 items
• Provide stable keys for list items (unique IDs, not index)
• Split state by update frequency
• Use concurrent features (useTransition, useDeferredValue) for responsiveness

After optimizing:

• Profile again to verify improvements
• Check bundle size reduction (if applicable)
• Ensure no regressions in functionality

Resources

• React Docs - Performance: https://react.dev/learn/render-and-commit
• React DevTools: Browser extension for profiling
• react-window: https://github.com/bvaughn/react-window
• Bundle analyzers: webpack-bundle-analyzer, rollup-plugin-visualizer
• Lighthouse: Chrome DevTools performance audit