• What - FID is a metric that measures a page's responsiveness during load. As such, it only focuses on input events from discrete actions like clicks, taps, and key presses.
  SPEC - https://wicg.github.io/event-timing/
  Blog - https://web.dev/fid/
• Performance Observer interface would let us capture this metric using the type first-input for supported browses
• We can also easily polyfill this metric for all browses, by adding listeners for click, keypress, mousedown and pointer events and measuring how long the handler took.

We can capture this for managed transactions such as page-load and route-change as both of them account for hard and soft navigations.