The FID is about how fast can user interact with the content, if the browser takes time to respond to user click, tap or scroll events then it is going to have higher FID and it will reduce user engagement over the time.
We would need to perform following operations:
You can learn more about code coverage tool here – https://developer.chrome.com/docs/devtools/coverage/
For a deep dive on how to improve FID, see Optimize FID. For additional guidance on individual performance techniques that can also improve FID, see: