Performance optimization for React applications — rendering, re-renders, data-fetching waterfalls, bundle size, and the JavaScript hot paths inside components. Use when the user mentions slow or janky renders, unnecessary re-renders, memoization (React.memo / useMemo / useCallback), lazy loading, code splitting, bundle size, request waterfalls, duplicate or repeated API requests, request deduplication, caching fetched data in localStorage, scroll or touch event listeners, laggy scrolling or typing, or asks to review, profile, or speed up React code. Not a general React authoring guide — for folder and layer structure use react-feature-sliced-design, for language and typing questions use typescript-development.