First Input Delay – FID FID measures the latency between the user’s first interaction with a page and the web page actually responding to that interaction. FID is most often affected by long execution times of large scripts, so try to minimize this as much as possible. A website will pass the FID rating if its FID is less than 100 milliseconds. Start by running the homepage and most important web pages test using Google PageSpeed Insights.
Here are some areas to look at to audit this signal
Opportunities to watch: Minimize main thread