Measure Before You Optimize
Premature optimization is the most common React performance mistake. Before adding a single memo, profile the application with the React DevTools Profiler and the browser performance panel to find where time is actually being spent.
Many "slow" apps are not slow because of rendering at all—they are slow because of oversized bundles, blocking network requests, or unoptimized images. Fix the real bottleneck, not the imagined one.
Beyond memo and useCallback
memo, useMemo, and useCallback are useful but frequently misapplied, adding complexity for negligible gains. They pay off most when an expensive child re-renders due to unstable props, not as a blanket policy.
Often the better fix is structural: lift state down to the component that needs it, split large components, or pass children as props so that a re-rendering parent does not force its subtree to re-render.
Rendering Large Lists and Heavy Trees
Rendering thousands of DOM nodes is a guaranteed performance problem. Virtualization libraries render only the visible window of a list, keeping the DOM small regardless of dataset size.
For expensive, non-urgent updates, React’s concurrent features—useTransition and useDeferredValue—let you keep the UI responsive by deprioritizing work that can wait.
Ship Less JavaScript
The fastest code is the code you never send. Route-based code splitting, lazy loading, and tree-shaking dramatically reduce the JavaScript the browser must parse and execute on first load.
Server components and streaming, where applicable, move work off the client entirely—often the single largest performance win available to a modern React application.