Developer Cheatsheets
How to Start Optimizing for React Performance

Cheatsheet: Assessing App Performance and Identifying Optimal Moments for Optimization in React

Hello, experienced React developers! Understanding when and how to optimize your application is critical. However, before embarking on that optimization journey, it's essential to measure the current performance of your application to establish a baseline. Here's your cheatsheet to identifying performance bottlenecks and determining the right time for optimization.

Measuring App Performance

  1. React Developer Tools (Profiling): This browser extension provides invaluable insights. Its Profiler feature helps identify performance bottlenecks in React components. It visualizes rendering times, helping you understand what's slowing down your app.

  2. Google Lighthouse: This open-source tool, built into Chrome DevTools, provides a comprehensive overview of the performance of your app. It gives performance metrics like First Contentful Paint (FCP), Speed Index, and Time to Interactive (TTI).

  3. Chrome DevTools Performance Tab: This records runtime performance, enabling you to see how your website performs during load and runtime, and helps identify costly JavaScript operations, long network requests, and more.

  4. Web Vitals: Web Vitals is an initiative by Google to provide guidelines on essential metrics for a healthy site. Metrics like Largest Contentful Paint (LCP), First Input Delay (FID), and Cumulative Layout Shift (CLS) are key to understanding user experience.

When to Consider Optimization

  1. Slow Component Rendering: If React DevTools indicates certain components are consistently slow to render, it's time to consider optimizing.

  2. Long Task Times: The Performance Tab in Chrome DevTools shows tasks in the main thread. If you observe long tasks (over 50ms), optimization might be necessary.

  3. Unsatisfactory Lighthouse Score: A low performance score (below 90) in Lighthouse is generally an indicator of needed optimizations.

  4. Poor Web Vitals Metrics: Poor LCP, FID, or CLS scores often indicate performance issues that negatively impact user experience.

The Path to Optimization

  1. Code Splitting: Implementing code splitting via React.lazy() and Suspense allows you to break up code and reduce the size of the initial load, increasing app performance.

  2. Memoization: Techniques such as using React.memo(), useMemo(), or useCallback() help avoid unnecessary re-renders or computations.

  3. Web Workers: For heavy computations, consider offloading tasks to Web Workers to free up the main thread.

  4. Using Production Build for Measurements: Always measure performance with a production build. Development mode includes extra warnings that can make React slower.

  5. Efficient Data Structures: Use appropriate data structures. For instance, using a JavaScript Set for membership checks is much faster than an array.

Okay great, but what does this look like in a practical setting? Here are some case studies showcasing the use of React Developer Tools and Google Lighthouse.

Case Study 1: Profiling with React Developer Tools

Consider this ExpensiveComponent which renders a list of items.

import React from "react";
const ExpensiveComponent = ({ items }) => {
  return (
      { => (
        <div key={}>{}</div>
export default ExpensiveComponent;

To profile this component:

  1. Install and open the React Developer Tools extension in your browser.
  2. Click on the "Profiler" tab in the React Developer Tools panel.
  3. Click the record button and interact with your application. For example, you can update the list of items that the ExpensiveComponent is rendering.
  4. Stop the recording and analyze the flamegraph and ranked chart. This provides an understanding of the components that take the most time to render.

Case Study 2: Auditing with Google Lighthouse

Let's use Google Lighthouse to audit an application that displays a list of items.

import React, { useState, useEffect } from "react";
const ItemList = () => {
  const [items, setItems] = useState([]);
  useEffect(() => {
      .then((response) => response.json())
      .then((data) => setItems(data));
  }, []);
  return (
      { => (
        <div key={}>{}</div>
export default ItemList;

To run a Lighthouse audit:

  1. Open your application in Google Chrome and press F12 to open Chrome DevTools.
  2. Navigate to the Lighthouse tab, select the "Performance" category, and then click "Generate report".
  3. Review the metrics provided. Lighthouse gives a score between 0 and 100 for performance. It also provides metrics like First Contentful Paint and Time to Interactive, which directly reflect the user's experience.
  4. The report also provides suggestions for performance improvements, such as removing unused code or reducing server response times.

In both these case studies, measuring the performance of the components will provide insights on where optimizations may be needed. It is always essential to measure before starting any performance optimization tasks.

Remember, premature optimization can lead to increased complexity and maintenance. The mantra is, "Measure, don't guess!" Always ensure the performance issue exists before optimizing. Happy coding!