Slow React apps hurt user experience and SEO, leading to higher bounce rates. The core issue is React's rendering behavior: parent state changes trigger re-renders for all child components, even if their props haven't changed. This fundamental mechanism is the primary source of performance bottlenecks in complex applications.

Developers can prevent unnecessary renders using React.memo for component memoization. For expensive calculations, useMemo caches results until dependencies change, while useCallback maintains stable function references. These hooks should be applied strategically after profiling identifies actual performance issues, not as premature optimization.

Code splitting with lazy loading reduces initial bundle size by loading routes and heavy components on demand. For massive lists, virtualization with libraries like react-window renders only visible items. State management optimization, such as splitting contexts by update frequency, prevents widespread re-renders. Always profile before and after changes using React DevTools to measure real impact.