First Input Delay (FID)
First-Input-Delay is a user-centric performance metric that measures the time between a user's first interaction with a webpage and the moment the Browser-Rendering-Engine is able to start processing that interaction. It is a critical component of the Core-Web-Vitals suite, specifically targeting the responsiveness and interactivity of a site during its initial load phase.
Technical Mechanism
The primary cause of a poor First-Input-Delay is heavy JavaScript execution. When the Main-Thread is occupied with parsing and executing large script files, it cannot respond to user inputs such as clicks, taps, or key presses. This creates a lag that can frustrate users and lead to higher bounce rates. While Lighthouse provides a proxy metric called Total-Blocking-Time, FID is a field metric that requires real user data, often collected via the Chrome-User-Experience-Report. Documentation on optimizing these interactions can be found at web.dev.
Thresholds and Benchmarks
To provide a good user experience, sites should strive to have a First-Input-Delay of 100 milliseconds or less. A delay between 100ms and 300ms is categorized as needing improvement, while anything above 300ms is considered poor. Developers can optimize these scores by minifying scripts, removing unused JavaScript, or implementing code-splitting. Further technical details are available on MDN Web Docs.
Transition to Interaction to Next Paint
In the evolving landscape of Web-Performance, Google has replaced FID with Interaction-to-Next-Paint (INP) as a Core Web Vital. INP is considered a more comprehensive metric because it measures all interactions on a page, whereas FID only measured the very first one. More details on the transition can be found at web.dev's blog.