FID (First Input Delay)

First Input Delay (FID) measures the time from when a user first interacts with a page (like clicking a link or button) to when the browser can actually begin processing that event. Google has transitioned to Interaction to Next Paint (INP) as the preferred metric. A good FID score is under 100 milliseconds. Between 100-300ms needs improvement, and over 300ms is considered poor. High FID occurs when the main thread is blocked by JavaScript execution. To improve FID/INP: minimize JavaScript execution time, break up long tasks, defer non-critical scripts, and use web workers for heavy computations.