Optimizing React Rendering Performance

React’s reconciliation algorithm is at the heart of how updates get applied to the DOM. Understanding it deeply gives you the power to write applications that feel instantaneous.

The Problem with Re-renders

Every time state changes in a React component, the entire component tree below it re-renders by default. For small apps, this is barely noticeable. But as your component tree grows, unnecessary re-renders compound.

function ExpensiveList({ items, filter }) {
  // This filters on every render, even if items haven't changed
  const filtered = items.filter(item => item.category === filter);
  
  return filtered.map(item => <Card key={item.id} {...item} />);
}

useMemo to the Rescue

function ExpensiveList({ items, filter }) {
  const filtered = useMemo(
    () => items.filter(item => item.category === filter),
    [items, filter]
  );
  
  return filtered.map(item => <Card key={item.id} {...item} />);
}

The React Compiler Changes Everything

The upcoming React Compiler automatically memoizes components and hooks, making manual useMemo and useCallback largely redundant. This is a paradigm shift in how we think about React performance.

Key Takeaways

• Profile before optimizing — don’t guess
• Use React DevTools Profiler to identify slow renders
• The React Compiler will automate most of this soon
• Focus on component architecture over micro-optimizations