---

name: performance-optimization description: Optimize React application performance with lazy loading, code splitting, memoization, and virtualization. Use when addressing slow renders, large bundles, or memory issues.

Performance Optimization

When to Use This Skill

Use when:

• Initial load time is too slow
• Components re-render unnecessarily
• Rendering large lists or data sets
• Bundle size is too large
• Memory usage is high

Code Splitting & Lazy Loading

React.lazy for Route-based Splitting

Copyimport { lazy, Suspense } from 'react';

const Dashboard = lazy(() => import('./pages/Dashboard'));
const Settings = lazy(() => import('./pages/Settings'));

function App() {
  return (
    <Suspense fallback={<LoadingSpinner />}>
      <Routes>
        <Route path="/dashboard" element={<Dashboard />} />
        <Route path="/settings" element={<Settings />} />
      </Routes>
    </Suspense>
  );
}

Named Exports with Lazy

Copy// For components not using default export
const LazyComponent = lazy(() =>
  import('./components').then(module => ({
    default: module.SpecificComponent
  }))
);

Preloading on Hover

Copyconst DashboardLazy = lazy(() => import('./Dashboard'));

function NavLink() {
  const preload = () => {
    import('./Dashboard'); // Starts loading on hover
  };

  return (
    <Link to="/dashboard" onMouseEnter={preload}>
      Dashboard
    </Link>
  );
}

Memoization

React.memo for Components

Copyinterface ExpensiveComponentProps {
  data: Data;
  onAction: (id: string) => void;
}

// Only re-renders when props change (shallow comparison)
export const ExpensiveComponent = React.memo(
  function ExpensiveComponent({ data, onAction }: ExpensiveComponentProps) {
    return <div>{/* expensive render */}</div>;
  }
);

// With custom comparison
export const CustomMemoComponent = React.memo(
  ExpensiveComponent,
  (prevProps, nextProps) => {
    return prevProps.data.id === nextProps.data.id;
  }
);

useMemo for Expensive Calculations

Copyfunction DataProcessor({ items, filter }: Props) {
  // Only recalculates when items or filter change
  const filteredItems = useMemo(() => {
    return items.filter(item =>
      item.name.includes(filter)
    ).sort((a, b) => a.name.localeCompare(b.name));
  }, [items, filter]);

  return <List items={filteredItems} />;
}

useCallback for Stable References

Copyfunction Parent({ items }: Props) {
  // Stable reference, doesn't change between renders
  const handleClick = useCallback((id: string) => {
    console.log('Clicked:', id);
  }, []); // Empty deps = never changes

  return (
    <>
      {items.map(item => (
        <MemoizedChild
          key={item.id}
          onClick={handleClick}
        />
      ))}
    </>
  );
}

List Virtualization

Using react-window

Copyimport { FixedSizeList } from 'react-window';

interface ItemData {
  items: Item[];
  onSelect: (id: string) => void;
}

function VirtualList({ items, onSelect }: ItemData) {
  const Row = ({ index, style }: { index: number; style: React.CSSProperties }) => (
    <div style={style} onClick={() => onSelect(items[index].id)}>
      {items[index].name}
    </div>
  );

  return (
    <FixedSizeList
      height={400}
      width="100%"
      itemCount={items.length}
      itemSize={50}
    >
      {Row}
    </FixedSizeList>
  );
}

Debouncing & Throttling

Copyfunction useDebounce<T>(value: T, delay: number): T {
  const [debouncedValue, setDebouncedValue] = useState(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebouncedValue(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);

  return debouncedValue;
}

// Usage
function SearchInput() {
  const [query, setQuery] = useState('');
  const debouncedQuery = useDebounce(query, 300);

  useEffect(() => {
    if (debouncedQuery) {
      search(debouncedQuery);
    }
  }, [debouncedQuery]);
}

Bundle Analysis

Copy# Vite
npx vite-bundle-visualizer

# Webpack
npx webpack-bundle-analyzer stats.json

Performance Checklist

• Use React.lazy for route-level code splitting
• Wrap expensive components with React.memo
• Use useMemo for expensive calculations
• Use useCallback for callback props passed to memoized children
• Virtualize lists with >100 items
• Debounce input handlers
• Lazy load images below the fold
• Split vendor chunks from app code