Interaction to Next Paint (INP): Understanding and Improving Core Web Vitals

Key Takeaways

• INP measures responsiveness by tracking the latency of all qualifying interactions during a page visit.
• A good INP score is 200 milliseconds or less.
• Optimizing INP involves minimizing input delay, processing time, and presentation delay.
• JavaScript execution often significantly impacts INP.
• Field data from real users provides the most accurate INP measurement.

Introduction: What is Interaction to Next Paint and Why it Matters

Ever click somethin’ on a webpage and it just kinda… sits there for a second? That waiting feeling, that’s what Interaction to Next Paint (INP) is all about measuring. It’s a Core Web Vitals metric, you see, replacing First Input Delay, which was alright but didn’t quite get at the whole story for user responsiveness. Why care? Because users hate waiting. A page that feels snappy, that responds instantly when you poke it, makes folks happier. It feels like the website’s payin’ attention to ya. Doesn’t everyone want that? Seems kinda obvious really.

Measuring INP: Tools and Metrics

How do you even know what your INP score *is*? That’s the next question, isn’t it. You gotta measure it, naturally. The best way? Real user monitoring, or field data as they call it. This captures actual user experiences, which is the true test. Lab tools, they can give you an idea, show you the potential problems, but the real world, with its messy connections and different devices, tells the final tale. Tools like PageSpeed Insights or the Chrome User Experience Report (CrUX) are your friends here. They collect and show you this field data. It is like seeing the matrix of user impatience, if you think about it that way. Don’t you want to see exactly where the waiting happens?

Deconstructing INP Latency: The Three Phases

So, when you interact, say, click a button, what precisely is INP measuring? It’s not just one thing; it is actualy three distinct parts. There is the input delay first off. This is the time from when you click until the browser can actually *start* processing that click. Then comes the processing time, where the website’s code does its thing, figuring out what your click is supposed to do. Finally, you have presentation delay. This is the time it takes after the code finishes for the browser to actually draw the result on the screen, showing you something changed. Each phase matters. If any one part is slow, the whole interaction feels laggy. It’s kinda like a tiny assembly line in your browser, every step gotta be quick.

What Affects INP? Common Causes

Many things can make an interaction slow down. What usually is the culprit? Big hunks of JavaScript running on the main thread, plain and simple. If the browser is busy executing a script, it can’t respond to your click immediately. This adds input delay. Or the script itself might take a long time to process your interaction, increasing processing time. Sometimes, heavy rendering work or complex layout shifts after the interaction cause presentation delay. Blocking resources, like render-blocking CSS or synchronous JavaScript, can also hold things up early on. Identifying these blockers is key. It is not always one big problem; sometimes many small things add up. Where is your page getting stuck? That’s the puzzle you need to solve.

Optimizing Interaction to Next Paint

Alright, you know your INP is bad. Now what? You gotta optimize for Interaction to Next Paint. The core idea is reducing the work the browser has to do. Can you break up long JavaScript tasks? Yes, often you can. Can you reduce the complexity of your event handlers? Make ’em run faster? Probably. Lazy loading non-critical resources helps ensure the page is responsive sooner. Deferring or asynchronously loading scripts is a classic move. Minimizing layout shifts post-interaction matters too; sudden movements take render time. Prioritize interactions over background tasks. It is a bit like decluttering a room before you can quickly move through it. Less stuff in the way means faster response.

INP and the Other Core Web Vitals

INP isn’t the only Core Web Vitals metric. There are Largest Contentful Paint (LCP), which measures loading performance, and Cumulative Layout Shift (CLS), which measures visual stability. Do they relate to INP? Absolutely. A page that loads slowly (bad LCP) might keep the main thread busy, impacting early interactions and thus INP. Significant layout shifts (bad CLS) can be triggered by interactions themselves or contribute to presentation delay. Optimizing one often helps the others. It’s a holistic picture of website performance, not just isolated numbers. You can’t just fix one thing and expect everything else to be magically fine. It all works together, like parts of a machine.

Expert Perspective on Interaction Performance

Thinking about interaction performance from a user’s perspective is vital. Someone clicking a button or typing in a form wants immediate feedback. They don’t care about your backend processes or frontend framework complexities. They just want the page to *do* the thing they asked it to do, right now. Experts often emphasize optimizing the critical user flows first. What actions do people take most often? Make those lightning fast. It is about the perceived speed as much as the technical measure. If the page feels responsive, even if the lab number is borderline, users are usually happy. Focusing on the *feeling* of performance, not just the score, is kinda overlooked sometimes.

Advanced INP Concepts and Considerations

Beyond the basics, INP measurement has nuances. What about cancelled interactions, like dragging something but letting go? Or long presses? The INP metric picks the *single* longest interaction latency for a page visit. Not an average, mind you. It’s the outlier that matters. Event delegation can sometimes complicate things; ensuring handlers are efficient is crucial. Using browser performance APIs like the Event Timing API lets developers get granular data on interaction timings. Understanding these details helps pinpoint exact bottlenecks. It is not always simple button clicks; scrolls, drags, typing – all can contribute. Knowing the specifics helps you figure out *exactly* what part of your page is slown down interactions the most. It is a deep dive, but needed for top performance.

Frequently Asked Questions about INP and Website Performance

What is a good INP score?

A good INP score, indicating your page is responsive for the majority of users, is 200 milliseconds or less. Scores between 200 and 500 ms need improvement, and anything over 500 ms is considered poor.

How does INP relate to overall website performance?

INP is a key measure of website performance, specifically focusing on responsiveness after the initial page load. It complements metrics that measure loading speed (like LCP) and visual stability (like CLS) to provide a more complete picture of user experience.

Does optimizing INP improve SEO?

Yes, because INP is part of Core Web Vitals, which is a ranking factor for Google Search. Improving your INP can contribute positively to your site’s search engine ranking, alongside providing a better user experience.

What is the biggest factor affecting INP?

Often, excessive JavaScript execution on the main thread is a primary cause of high INP latency. Long tasks block the browser from responding quickly to user interactions.

Should I focus on field data or lab data for INP?

Field data (like from CrUX or RUM tools) is the most important as it reflects real user experience. Lab data is useful for debugging and identifying potential issues during development, but field data is the true measure of your site’s INP performance.