Why is FID not part of lab data/Lighthouse?

Lab tools such as Lighthouse (or PageSpeed Insights, as PageSpeed Insights uses Lighthouse engine) can’t measure FID since there are no end-users to calculate interactions. Lab tools could simulate an input, but every user has a different experience depending on their constraints and thus also have another perception of performance. This matters as different users might start interacting at different intervals, resulting in different outcomes.

Instead of trying to simulate FID, these tools use a metric called Total Blocking Time (TBT) to predict potential FID bottlenecks. One of my articles might help to visualize FID bottlenecks. Do note the web performance working group is working on improving FID.