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Slow React rendering usually comes from unnecessary re-renders, large component trees, or unmemoized computations. Profile the component tree, apply memoization where props are stable, and split large bundles with code splitting.

Next.js performance improves by choosing the right rendering strategy (SSR, SSG, or client-side), reducing client-side JavaScript, and optimizing data fetching. Review hydration costs and move heavy logic to server components where possible.

WebAssembly fits compute-intensive tasks like image processing, simulations, or parsing large datasets where JavaScript is too slow. For typical UI logic and DOM work, plain JavaScript or TypeScript remains the better choice.

Excessive re-renders typically result from unstable prop references, inline object or function creation, or misplaced state. Stabilize references with useMemo and useCallback, and lift or colocate state appropriately.

Common mistakes include missing effect dependencies, stale closures, and forgetting cleanup functions for subscriptions or timers. Always declare all dependencies in useEffect and return cleanup functions for side effects that allocate resources.