First Input Delay (FID)

What Is First Input Delay?

Although FID was replaced by INP in March 2024, the underlying optimization principles remain identical: reduce blocking JavaScript, use code splitting, and keep the browser’s main thread free. INP is the stricter metric because it measures not just the first interaction but all interactions throughout the entire page visit. Even if you had good FID scores, check your INP score regardless.

First Input Delay (FID) was a Core Web Vital metric from Google that measured the delay between a user interaction (e.g., a click or key press) and the browser’s response. A high FID indicates that the browser is blocked by heavy JavaScript tasks and cannot respond quickly to user input — leading to frustration. However, FID was replaced by INP (Interaction to Next Paint) in March 2024. The metric is no longer current, but it remains relevant for understanding technical SEO discussions.

Technically, FID was caused by blocking JavaScript that kept the browser’s main thread busy. While Google parses, compiles, and executes JavaScript, the browser cannot process user actions. This was particularly problematic on mobile devices with weaker processors. Google calculated FID as the time between when a user interacts (e.g., clicks a button) and when the browser actually begins processing that action. An FID under 100 milliseconds was optimal, 100–300ms needed improvement, and over 300ms was poor.

Although FID was replaced by INP, the optimization approaches are similar and still relevant: reduce JavaScript quantity and size through code splitting and asynchronous loading. Use Web Workers to offload heavy computations from the main thread. Implement long-task optimization and set priority hints (e.g., fetchpriority=“high”). Monitor INP now instead of FID with Google PageSpeed Insights. FID was particularly relevant on mobile and emphasized the importance of keeping JavaScript overhead low — these principles still apply to user experience and thus to rankings.