How to Debug and Improve React Performance

Modern React applications are powerful — but that power often comes with hidden costs.
As the project grows, the app can become sluggish, unresponsive, or bloated, even if the codebase looks clean.
The main cause? Oversized bundles, unnecessary re-renders, expensive computations, and unoptimized component structures.

How to deal with them? Here are the tools every frontend developer should know and use regularly.

1. Measure with the Network Tab

• Open Chrome DevTools → Network.
• Disable cache and reload the page.
• Inspect the waterfall — look for large or slow-loading files.
• Check TTFB (Time to First Byte). If it’s high, your backend or CDN is slow.
• Check Content Download times — large JS bundles are a common cause of slow first load.
• Test with “Fast 3G” throttling to simulate real user conditions.

Goal: Identify large payloads or long waiting times.

2. Analyze Execution with the Performance Tab

• Open Chrome DevTools → Performance.
• Click Record, reload the page, then Stop.
• Watch the Main thread — long purple or yellow blocks mean the browser is busy executing JavaScript or rendering.
• Focus on Script Evaluation and Rendering phases.
• Look for Long Tasks (>50 ms) — they block the UI.
• Hover over spikes in the timeline to see which scripts or components cause them.

Goal: Find scripts or components that keep the main thread busy for too long.

3. Audit Overall Performance with Lighthouse

• Open Chrome DevTools → Lighthouse.
• Choose the Performance category (optionally others).
• Run the test in Incognito mode with Throttling enabled for realistic results.
• Review key metrics:
  • LCP (Largest Contentful Paint) — how fast main content appears.
  • INP (Interaction to Next Paint) — how responsive interactions feel.
  • CLS (Cumulative Layout Shift) — how stable layout is.
  • TBT (Total Blocking Time) — how much JS blocks the main thread.

Lighthouse provides a macro view — an overview of the app’s performance health.
Use it to spot systemic issues after addressing specific bottlenecks found with Network and React DevTools.

Goal: Get measurable performance scores and identify global improvements.

4. Inspect Rendering with React DevTools

• Install the React Developer Tools extension.
• Open the Components tab and enable Highlight updates when components render.
  Interact with the app and watch which components flash — those are re-rendering.
• Inspect props and state of components that re-render often.
  If nothing significant changes, wrap them with React.memo or stabilize props using useCallback / useMemo.
• Switch to the Profiler tab.
  • Click Start profiling, interact with your app, then Stop.
  • Review the flamegraph to see which components take the longest to render.
  • Enable Record why each component rendered to see whether it was triggered by state, props, or context changes.

Goal: Detect components that render too often or render too slowly.

5. Fix What You Found

Once you know the cause:

• Reduce bundle size with lazy loading or by removing heavy libraries.
• Memoize stable components and handlers.
• Avoid global context that triggers widespread re-renders.
• Move heavy computations outside render functions.

By combining Network, Performance, Lighthouse, and React DevTools, you’ll gain both a macro and micro understanding of your app’s performance — and a clear path to optimize it.