描述
Run measurement-led React and Next.js performance audits for waterfalls, bundle size, RSC/server-client boundaries, hydration mismatch, rerender churn, script cost, and slow page interactions. Use when the user needs help diagnosing or refactoring a slow React UI, App Router route, heavy client component, or Next.js page with excess JavaScript or weak client/server boundaries. Triggers on: React perf, Next.js perf, waterfall, bundle size, hydration, rerender, client component too heavy, slow route, React profiler, web vitals.