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INP Optimization for React and Next.js

Emily Harris
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When building modern web applications with React and Next.js, user experience is a top priority. One of the most recent metrics introduced by Google to measure interactivity and perceived performance is the Interaction to Next Paint (INP). As the web evolves, optimizing for INP is becoming essential for developers who care about fast, responsive applications.

INP is part of Google’s Web Vitals initiative and is currently considered an experimental metric, but it is poised to become a key part of the Core Web Vitals. Therefore, understanding and optimizing INP will help developers future-proof their applications.

What Is INP?

Interaction to Next Paint measures the time between a user interaction — such as a click, tap, or keypress — and the next time the browser visually updates in response. Unlike First Input Delay (FID), which only focuses on the delay before the event handler starts execution, INP covers the complete cycle: the input delay, event handling, and the final visual update.

This makes INP a more comprehensive metric for assessing a user’s true interactive experience. High INP values indicate sluggish applications where users may feel that their actions aren’t being registered.

Why INP Matters in React and Next.js

React and Next.js are powerful tools for building interactive UIs. However, their power comes with complexity, which can sometimes impact performance. Because React applications can involve frequent re-renders and large component trees, optimizing for INP is crucial to maintain responsive interactions.

Next.js adds another layer, as it includes server-side rendering (SSR), static generation (SSG), and APIs like getServerSideProps and getStaticProps. While these can improve page loading speed, they don’t directly impact INP. Instead, how you manage client-side state and interactivity will play a bigger role.

Key Causes of Poor INP in React Apps

To address INP problems effectively, it’s important to understand the most common causes of high INP scores:

How to Measure INP

You can measure INP using several tools:

To track INP with the Web Vitals library:

import { onINP } from 'web-vitals';

onINP((metric) => {
  console.log(metric.value);
});

Best Practices for INP Optimization

INP optimization often comes down to smart component management and reducing unnecessary work on the main thread. Below are specific strategies you can apply in React and Next.js apps:

1. Split Your Code Effectively

Large JavaScript bundles can delay event handling and visual updates. Implementing code-splitting ensures that only the code needed for initial render is loaded, deferring other parts until user interaction requires them.

Next.js makes this easier with dynamic imports:

import dynamic from 'next/dynamic';

const DynamicComponent = dynamic(() => import('../components/HeavyComponent'));

2. Use Memoization

React provides hooks like useMemo and useCallback to prevent unnecessary re-renders and recalculations:

const memoizedValue = useMemo(() => computeExpensiveValue(data), [data]);

Be cautious: overuse of memoization can make code harder to maintain, so profile before optimizing.

3. Use the Concurrent Features in React 18+

React 18 introduces concurrent rendering, allowing React to interrupt long rendering tasks to keep the interface responsive. Use features like startTransition when updating state for non-urgent updates:

import { startTransition } from 'react';

startTransition(() => {
  setState(newValue);
});

4. Defer Non-Critical Work

Heavy computations, API calls, and unnecessary state updates should not be performed during event handlers. Instead, consider queuing them with requestIdleCallback or lazy loading:

requestIdleCallback(() => heavyComputation());

5. Optimize Image Loading and Usage

Heavy images or poorly optimized media assets can delay paints. Use Next.js’s built-in Image component to serve responsive and optimized images:

import Image from 'next/image';

<Image src="/image.jpg" alt="Example" width={500} height={300} />

6. Keep Event Handlers Lean

Your React event handlers should remain focused. Avoid doing too much in a single click handler, and always aim to separate UI updates from business logic operations.

7. Profiler API and DevTools

Use React’s built-in profiler with React DevTools to track unnecessary renders and identify slow components:

  1. Open your app in development mode.
  2. Use the React Profiler tab in DevTools.
  3. Record interactions and identify performance bottlenecks.

Monitoring and Feedback Loop

Optimization is an ongoing process. Consider integrating performance monitoring tools like:

By establishing a feedback loop, you can adapt your architecture as your app scales and user interactions increase in complexity.

Conclusion

INP is not just another performance score to chase; it reflects your app’s ability to feel instant and responsive. In React and Next.js applications, optimizing for INP requires a combination of smart architecture, efficient event handling, and cutting-edge tools.

Understanding the lifecycle of user interaction — from event trigger to paint — allows developers to build experiences that not only look great but *feel* intuitive and responsive. As web standards continue to evolve, having a strong INP performance will soon be as essential as having a quick initial load time.

Start optimizing today, and ensure your users are never left waiting after they click.

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