I hate slow websites. They are annoying to use and frustrating to work on. But what does it mean to be “slow”? It used to be waiting for document load. Then waiting for page ready. But with so many asynchronous patterns in use today, how do we even define what “slow” is? The W3C has been working on this with the new Event Timing and Element Timing API, and has defined some new Web Vital metrics to describe the different ways that slow performance can impact a webpage.

First Input Delay (FID) measures how long the browser took to respond to the first user input event. It’s one of several Core Web Vital metrics that measure how real users perceive the performance of modern web applications. New measurements like First Input Delay are increasingly important as JavaScript and SPA’s do more work on the client side.