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Interaction to Next Paint fully replaced First Input Delay as Google's official Core Web Vitals responsiveness metric in March 2024, after a period running alongside it as an experimental metric. INP measures the latency of all interactions throughout a page's visit, not just the very first one — a meaningfully more complete picture of real responsiveness.
Why FID was replaced
First Input Delay only measured the delay before the browser began processing a visitor's very first interaction — a narrow, if useful, signal. It said nothing about whether the tenth click on a page, deep into a session, felt responsive. INP measures interactions across the entire page visit and reports a representative worst-case-ish value, giving a far more complete responsiveness picture.
- FID measured only the first interaction's initial delay.
- INP measures the full latency of essentially all interactions throughout the visit.
- INP reports a high-percentile value across all measured interactions, better reflecting the worst typical experience.
Google's scoring thresholds
Google considers an INP of 200 milliseconds or less 'good', 200 to 500 milliseconds 'needs improvement', and above 500 milliseconds 'poor', measured at the 75th percentile of real user interactions.
INP does not care about your fastest interaction — it is built to surface the interactions that actually frustrated a real visitor, which is a much harder, more honest thing to optimise for.
What typically causes poor INP
Heavy JavaScript execution that blocks the main thread, large third-party scripts (analytics, chat widgets, ad tags) competing for processing time, and complex, expensive event handlers triggered by common interactions like clicking a button or opening a menu, are the most common causes.
- Long-running JavaScript tasks that block the browser's main thread.
- Heavy third-party scripts — analytics, chat, advertising — competing for processing resources.
- Inefficient event handlers on common interactions like clicks, taps and menu toggles.
- Large, unoptimised DOM structures that make every interaction more expensive to process.
Practical fixes
Audit and reduce third-party script load specifically, break up long JavaScript tasks so the browser can respond to interactions between them, and simplify overly complex DOM structures where possible. Chrome DevTools' Performance panel and web.dev's INP guide are the most reliable diagnostic starting points.
Frequently asked questions
When exactly did INP replace FID as an official Core Web Vital?
INP became the official responsiveness metric on March 12, 2024, after running as an experimental metric alongside FID for roughly a year beforehand.
Can a site with good FID have poor INP?
Yes, this is actually common — a site can handle its very first interaction quickly (good FID) while struggling with responsiveness later in the visit as more scripts load and the page becomes more complex (poor INP).
Is INP harder to optimise than FID was?
Generally yes, since it requires addressing responsiveness across the entire page lifecycle rather than just the initial load moment, often requiring a broader review of scripts and interaction handling.
If your site's INP scores need attention, get in touch with our team for a technical review.
The takeaway
Audit your third-party scripts specifically for INP impact — heavy analytics, chat widgets and ad scripts are the most common, and most fixable, cause of poor responsiveness.
Written by Fibvo — Bengaluru and Erode, India — working with clients in other time zones. Questions, or think we have it wrong? Tell us.
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