Lead with measurement: "I open React DevTools Profiler, record an interaction, and look for components that re-render without their props changing. Then I apply React.memo or restructure the component tree."
Strong answers follow a systematic approach: use React DevTools Profiler to identify unnecessary re-renders, check component tree depth, look for missing keys in lists, and identify expensive computations. Solutions: React.memo for pure components, useMemo/useCallback for expensive calculations and stable references, code splitting with React.lazy, virtualisation for long lists. Best candidates measure before optimising.
Senior-level question. Developers who immediately reach for memo without profiling are cargo-culting. Those who can systematically diagnose re-render chains and apply targeted fixes are strong.