What Is Interaction to Next Paint (INP)?
Interaction to Next Paint is a Core Web Vital metric that measures a website's responsiveness to user interactions. Introduced by Google in 2024 to replace First Input Delay (FID), INP provides a more comprehensive view of how quickly users receive visual feedback when they interact with your site.
The key insight behind INP is that responsiveness isn't just about how quickly your code starts processing an interaction--it's about how quickly users see the results. An interaction has three components:
- Input Delay: The time between a user's input and when the browser begins processing it
- Processing Duration: How long the event handlers take to execute
- Presentation Delay: The time needed for the browser to render the next frame with visual updates
INP captures the total of all three, giving you a complete picture of the user experience. For websites built with AI-powered features, optimizing INP is especially critical since complex model inference can easily block the main thread if not properly managed.
INP Performance Thresholds
200ms
Good INP Score
500ms+
Poor INP Score
3
Components to Optimize
Understanding the Three Components of INP
To optimize INP effectively, you need to understand what contributes to it. Each component requires different optimization strategies.
Input Delay
Input delay occurs when user input is queued behind other work the browser is doing. This typically happens during long JavaScript tasks that block the main thread. Even if your event handlers are perfectly optimized, input delay can still occur if the browser is busy parsing, compiling, or executing other scripts.
Common causes of input delay:
- Large JavaScript bundles being parsed and compiled
- Third-party scripts running on your page
- Long tasks from any source blocking the main thread
- Multiple rapid interactions that queue up
Processing Duration
Processing duration measures how long your event handlers take to execute. This is the part of INP you have the most control over. Complex event handlers, excessive DOM manipulation, and synchronous operations all contribute to longer processing duration.
Presentation Delay
Presentation delay is the time between when your event handler finishes and when the browser actually paints the next frame. Even if your JavaScript executes instantly, the browser may need additional time to calculate layouts, compositing layers, and actually render pixels to the screen.
Understanding these components helps you target your web development efforts where they'll have the most impact on user-perceived performance.
Measuring INP: Tools and Techniques
Chrome DevTools Performance Panel
The most direct way to analyze INP is through Chrome DevTools. Open any page, navigate to the Performance tab, and record a session while interacting with the page. Look for the "Interaction to Next Paint" entries in the Timings track, which show exactly how long each interaction took.
Chrome DevTools highlights interactions over 200ms in yellow and those over 500ms in red, making it easy to identify problematic interactions at a glance.
Lighthouse
Lighthouse provides lab-based INP measurement, which is useful for testing during development and CI/CD pipelines. Run Lighthouse in Chrome or PageSpeed Insights to get an INP score along with specific recommendations for improvement.
Field Data with the Web Vitals JavaScript Library
For real-world INP data from actual users, Google provides the web-vitals JavaScript library. This library works with the Chrome User Experience Report (CrUX) to collect and report field data.
Why field data matters:
- It reflects real devices, networks, and usage patterns
- It captures interactions that lab tests might miss
- It shows you how your actual users experience your site
Implementing comprehensive performance monitoring allows you to track these metrics continuously and identify regressions before they impact your search rankings.
Optimization Strategies for Better INP
Break Up Long Tasks
The single most effective INP optimization is breaking up long tasks. When JavaScript execution blocks the main thread for more than 50 milliseconds, it becomes a "long task" that delays all other work, including responding to user input.
Optimize Event Handlers
Event handlers should do the minimum necessary work and defer everything else. The principle is simple: update the UI immediately, then do everything else asynchronously.
Avoid Layout Thrashing
Layout thrashing occurs when JavaScript reads layout properties and then writes to the DOM, forcing the browser to recalculate layout synchronously. Batch all reads first, then all writes.
Use requestAnimationFrame
When updating the DOM based on user input, use requestAnimationFrame to ensure updates happen at the optimal time in the browser's rendering pipeline.
Defer Non-Critical JavaScript
Load only the JavaScript needed for the current view. Code splitting, lazy loading, and deferring non-critical scripts all help reduce initial load time.
These techniques are fundamental to building high-performance web applications that deliver exceptional user experiences.
Code Example: Breaking Up Long Tasks
1// Instead of processing everything synchronously2function processLargeData(data) {3 for (const item of data) {4 expensiveOperation(item);5 }6}7 8// Break work into smaller chunks with yield points9async function processLargeData(data) {10 const chunkSize = 50;11 for (let i = 0; i < data.length; i += chunkSize) {12 const chunk = data.slice(i, i + chunkSize);13 await processChunk(chunk);14 // Yield to main thread between chunks15 await new Promise(resolve => setTimeout(resolve, 0));16 }17}18 19// Optimized event handler pattern20button.addEventListener('click', async (event) => {21 // 1. Update UI immediately for perceived responsiveness22 updateButtonState(event.target);23 24 // 2. Yield to let the browser paint25 await new Promise(resolve => requestAnimationFrame(resolve));26 27 // 3. Do expensive work after paint28 await saveData();29 await updateAnalytics();30});INP Optimization for AI-Powered Applications
AI-powered applications often face unique INP challenges due to:
- Large model weights that must load before interaction
- Inference operations that can block the main thread
- Streaming responses that update the UI incrementally
Strategies for AI Applications
Offload Model Inference to Web Workers: Run AI inference in web workers to keep the main thread responsive during processing.
Stream Results Incrementally: Rather than waiting for complete AI responses, stream partial results to the UI as they become available.
Implement Progressive Enhancement: Ensure basic functionality works without AI features, then load and initialize AI capabilities after the page is interactive.
Use Caching Aggressively: Cache model outputs and intermediate results to avoid redundant computation.
Our team specializes in building AI-integrated web applications that maintain excellent Core Web Vitals while delivering powerful intelligent features. Contact us to learn how we can optimize your AI-powered experience.
Common INP Pitfalls and How to Avoid Them
Over-Responsive Event Handlers
Attaching event handlers to every mousemove or scroll event can overwhelm the browser. Use techniques like debouncing, throttling, or passive event listeners.
// Throttle scroll handlers
window.addEventListener('scroll', throttle(() => {
updateScrollIndicators();
}, 100));
Hydration Delays in SPAs
Single-page applications often block interactivity until hydration completes. Consider partial hydration, island architecture, or resumable patterns.
Expensive Computations in Event Handlers
Moving heavy computations out of event handlers and into web workers or cache-as-you-go patterns can dramatically improve INP.
Avoiding these pitfalls is essential for maintaining fast page load times and ensuring users can interact with your site immediately.
Building an INP Monitoring Strategy
Sustainable INP optimization requires ongoing monitoring:
- Set up RUM collection using the web-vitals library
- Establish performance budgets for INP (target: p95 < 200ms)
- Create dashboards to track INP trends over time
- Implement regression testing in CI/CD using Lighthouse
- Audit third-party scripts regularly for performance impact
By integrating performance monitoring into your development workflow, you can catch INP regressions early and maintain excellent Core Web Vitals scores that support your search engine optimization efforts.
Break Up Long Tasks
Divide large JavaScript operations into smaller chunks that yield to the main thread, keeping it responsive to user input.
Minimize Event Handler Work
Update the UI immediately in handlers, then defer expensive operations to run after the next paint.
Batch DOM Operations
Read all layout properties first, then perform all writes together to avoid layout thrashing.
Use requestAnimationFrame
Schedule visual updates during the browser's optimal painting window for smoother animations.
Monitor with RUM Data
Collect real-world INP metrics to understand how actual users experience your site.
Audit Third-Party Scripts
Evaluate and optimize third-party code, which is often the biggest source of INP problems.