Parallel quicksort algorithm taking way too long
The problem seems to be the fact that you are not dealing with threads (as you should), but rather entire processes (which is more computationally demanding). Refer to this page for details on how to spawn threads.

Also, as a side node, it makes no sense to keep spawning new threads over and over again, since spawning a thread can take easily a couple of milliseconds, which would obliterate all performance gain. Instead, ask the user to pass the maximum amount of threads allowed, and do not spawn any more than that. Furthermore, it would be reasonable to cancel spawning a new thread in case the range to sort it receives is too small.

Hope that helps.