It is all moot if one (or more) of the threads is hogging I/O.

Update 1: while I make a point not to comment on downmodded posts of mine, I have to say that i/o contention among threads is a SERIOUS issue - even on an SMP machine that has multiple cores/cpus. So put that in your pipe and smoke it.

Update 2: I've been asked by BrowserUk to clarify, so here we go. As it was pointed out, threading simply divides time among related processes.

On a single CPU system, the issue of a thread hogging i/o is indistinguishable from a simple blocking thread - sibling threads simply won't get a chance to execute until the thread that is executing finishes with its reads/writes.

On shared memory (multi-core, SMP), the situation changes. Multiple threads may be executing, but if some small subset of the threads are engaging in heavy i/o, things will come screeching to a crawl. My point is that there is an additional contention for i/o bandwidth, and unless there is a high performance disk subsystem available to your threads, simply thowing more CPUs at it won't make a lick of difference - and could in fact make your performance a lot worse than just running the code serially.

We don't know what the OP's code is doing. If it is reading and writing a lot of data, this could have a significant impact on the performance of the threads.