FID is a metric used to track a page’s interactivity. Interactive websites are good websites. But websites that prompt interactivity need to be markedly efficient in handling button clicks and requests. From the time users initially dynamically interact with your page (click a button, for instance) to the time that your site begins to process the request, FID is the name of the game. To provide a good user experience, your website should have an FID score on all pages of 100 milliseconds or less.