First Input Delay measured the gap between when a visitor first interacts with your page (clicking a button, tapping a link) and when the browser actually starts processing that interaction. Good FID was under 100ms. Above 300ms, and visitors feel like the page is frozen. Google included FID as a Core Web Vital from 2020 until March 2024, when it was replaced by INP (Interaction to Next Paint).
FID's problem was that it only measured the first interaction. A page could score perfectly on FID because the first click was responsive, then become completely sluggish on subsequent interactions. INP fixes this by measuring responsiveness across all interactions during the entire page session.
Why FID still matters conceptually
Even though FID is deprecated as an official metric, the underlying principle is critical: when someone clicks your CTA and nothing happens for 300ms, they click again. And again. Then they leave. On landing pages, input delay usually comes from the main thread being blocked by JavaScript — analytics scripts, A/B testing tools, chat widgets, and tracking pixels all competing to execute.
The fix hasn't changed with the metric name: reduce JavaScript execution time, break up long tasks, defer non-critical scripts, and use web workers for heavy computation. If your page has more than 500ms of total blocking time, your interactions are probably sluggish regardless of which metric you're measuring.