Debugging INP: from DevTools to RUM Data

INP (Interaction to Next Paint) is one of the Core Web Vitals metrics and a critical signal for measuring how users actually experience your site. Unlike metrics that focus on loading or stability, INP reflects responsiveness over time. It measures how long it takes for the page to visually respond after a user interaction, like a click or a tap.

If you've ever clicked a button and felt like nothing was happening for what seems like an eternity, you've experienced poor INP. And the truth is, many websites are slower here than they realize.

What's really happening when INP is slow?

Let's break down the moving parts of an interaction, sometimes visualized as whiskers in DevTools:

• Input delay → The time between a user action (click, keypress, tap) and the browser starting the event handler.
  Common culprit: long tasks blocking the main thread.
• Processing time → The actual time spent running the event handler(s).
  Common culprit: heavy JavaScript logic, multiple chained handlers, lack of throttling or debouncing.
• Presentation delay → The time from when the handler finishes until the browser is able to paint the next frame.
  Common culprit: layout thrashing, forced reflows, expensive style recalculations, or paint blocking animations.

Each phase needs its own optimization strategy:

• Reduce input delay with code splitting, breaking up long tasks, and web workers.
• Reduce processing time by trimming event handler logic.
• Reduce presentation delay by optimizing rendering paths and animations.

Debugging locally: DevTools to the rescue

When starting an INP debugging session, Chrome DevTools is your friend:

1. Record a performance profile while interacting with your site.
2. Look for "long tasks" that block the main thread.
3. Map user actions (clicks, taps) to the scripts executing at that moment.
4. Identify if delays come from input, processing, or rendering.

This gives you a clear local picture, but it doesn't tell you what your real users are experiencing.

Debugging at scale: using RUM and LoAF data

That's where Real User Monitoring (RUM) tools come in. For example, RUMvision explains how to use LoAF (Long Animation Frames) to connect what you see in the lab with what's happening in the wild.

While Chrome DevTools only shows you file sizes of third party scripts, LoAF reveals their impact on responsiveness which is a much more useful perspective. This helps prioritize whether that analytics or chat widget is dragging your INP score down for actual users.

As the RUMvision team puts it, INP debugging isn't just about spotting what is slow, it's about knowing what matters most to your users and tackling it in the right order.

Practical tips for faster INP

• Audit third party scripts: Don't just look at size, measure their runtime impact.
• Trim event handlers: Keep them lean and push heavy work to background tasks or workers.
• Optimize rendering: Use GPU friendly animations, avoid layout thrashing, and minimize recalculations.
• Focus on real user interactions: Debug the actual elements users click on most, not just what's easiest to test locally.

Wrapping up

Debugging INP can feel overwhelming at first. Every millisecond of delay can come from a different part of the interaction pipeline. But by combining DevTools for local deep dives and RUM data (with LoAF) for real world validation, you can uncover the true bottlenecks hurting your users.

The goal isn't to chase a perfect score, it's to make your site feel fast and responsive where it matters most. Because in the end, a snappy site isn't just better for Core Web Vitals, it's better for your users.

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This is a guest post by Jordy Scholing, Co-owner & Customer Success Lead at RUMvision. Learn more about Real User Monitoring and performance optimization at RUMvision.