Why is the FID metric part of the Core Web Vitals?

The timedelay between the (first) moment of user interaction and the moment the browser was able to respond to the user interaction, is encapsulated in the First Input Delay (or FID) metric. Responses within 100ms are perceived as fast by the human eye. Slower response times on user interactions might be experiences as an unnatural delay, leading to distrust or user frustration.