Auditing Performance

Performance Audit

Use this skill when the user asks to optimize performance, reduce load times, fix slow pages, or audit Core Web Vitals.

Steps

1. Analyze bundle size

   • Run npx @next/bundle-analyzer (Next.js) or npx vite-bundle-visualizer (Vite) to identify large dependencies.
   • Look for large libraries that could be replaced with lighter alternatives (e.g. moment → date-fns, lodash → individual imports or native methods).
   • Check for duplicated dependencies in the bundle.
   • Verify tree-shaking is working (no barrel file re-exports pulling in unused code).

2. Audit rendering performance

   • Identify components that re-render unnecessarily — look for inline object/array/function creation in JSX props.
   • Check for expensive computations in render paths that should use useMemo.
   • Verify lists use proper key props (not array index for dynamic lists).
   • Look for layout thrashing (reading DOM measurements then writing styles in a loop).

3. Check data fetching

   • Identify request waterfalls — sequential API calls that could be parallelized with Promise.all.
   • Look for data fetched on the client that could be fetched on the server.
   • Check for missing pagination on large data sets.
   • Verify API responses aren't over-fetching (returning fields the client doesn't need).

4. Database query optimization

   • Look for N+1 query patterns (a query per item in a list).
   • Check for missing indexes on columns used in WHERE, ORDER BY, and JOIN clauses.
   • Identify queries that could use SELECT with specific columns instead of SELECT *.
   • Look for missing connection pooling.

5. Check assets

   • Verify images use modern formats (WebP/AVIF) and are properly sized.
   • Check for missing loading="lazy" on below-the-fold images.
   • Verify fonts use font-display: swap and are preloaded.
   • Check for render-blocking CSS or JavaScript.

6. Generate recommendations — produce a prioritized list of optimizations ranked by impact (High / Medium / Low) with estimated effort for each.

Notes

• Focus on measurable improvements — use Lighthouse, WebPageTest, or the Performance tab in DevTools.
• Don't prematurely optimize — profile first, optimize bottlenecks.
• For React apps, use React DevTools Profiler to identify slow components.