Interaction To Next Paint (INP) is a critical user-centric performance metric used to track a web page's responsiveness. It measures the delay between a user's interaction with a page, such as clicking a link or tapping a button, and the time the browser can begin processing event handlers in response. The INP is essential because it directly impacts the user's perception of the site's interactivity and smoothness.

A lower INP score means a more responsive site, which matters because responsiveness directly impacts user satisfaction. A webpage that reacts swiftly to user inputs feels more fluid and natural, improving the overall user experience.

Taking care of the INP is beneficial for keeping users engaged and also affects the site’s technical optimization. Since search engines prioritize the user experience, a better INP score can contribute positively to a site's search engine optimization (SEO).

How to optimize INP?

Optimizing Interaction to Next Paint (INP) in Next.js is key to making your site feel more responsive and user-friendly. Here are some practical ways to improve it in your app:

• Selective Hydration with Suspense:

Use React 18's selective hydration features to make parts of your page interactive without loading the entire JavaScript bundle. Wrap major components within a <Suspense> boundary to enable non-blocking hydration.

• Lazy Loading:

Use lazy loading for images, scripts, and components that aren’t essential to the initial user interaction. Next.js supports automatic image optimization and lazy loading with the <Image> component from next/image.

• Optimize JavaScript Bundling:

Code-splitting with dynamic imports helps reduce JavaScript bundle size by loading only what's needed, which lightens the load on the main thread.

• Use CSS for animations:

Where possible, use CSS instead of JavaScript for animations and transitions. CSS animations are handled by the browser's compositor thread, which can help keep the main thread free for other tasks. This approach optimizes CSS animations for common animation needs like hover effects, transitions between states, or simple movements.

• Throttle and Debounce Event Handlers:

For events that trigger frequently (like scrolling or resizing), implement throttling or debouncing to limit the number of times event handlers are called. Thanks to that, you'll reduce the workload on the main thread.

• Reduce DOM Complexity:

Simplify the DOM structure and keep it lean. A complex DOM can slow down the page, requiring more computational power to render and re-render UI components.

• Use Web Workers:

Offload heavy computations to web workers. It helps to keep the main thread clear for UI and interaction handling, which is critical for maintaining a low INP.

• Implement Efficient Data Fetching:

Optimize data fetching strategies using SWR or React Query for efficient data caching and re-fetching, which helps keep the UI responsive.

• Prioritize Content and Interactions:

Use the startTransition API to mark certain updates as non-urgent. This allows React to interrupt these updates if more critical tasks (like handling user inputs) need attention.