What is threadpool?

since the no of req and rounds of hashing were constant in each case, increasing thread size increases time taken coz it is now executing parallely. Using one thread we were clocking less time ,using more threads we are increasing our time, why would we want that ? We want the time to be least ,isn't it? Are there other factors involved?

@Codedamn The main code stack itself would take one thread right. So then in the default case don't we only have three threads for allocation ?