Increasing Thread pool for best perfomance

So, i am doing a little side project that has a lot of File I/O. I increased tha maximum threads in node thread pool from the default four up to eight and saw a significant improve on peformance. I know that if you increase too mutch you will end up with peformance loss due to thread context switching overhead (or smthng like that, correct me if i'm wrong!). So how do i know exactly how much threads will do the work?