Apple's Threading Programming Guide states that:
Although good for occasional communication between threads, you should not use the performSelector:onThread:withObject:waitUntilDone: method for time critical or frequent communication between threads.
Which begs the questions: Which is, then, the acceptable method for frequent inter-thread communication, and why is performSelector:onThread:withObject:waitUntilDone: specifically not recommended.
ps: Not waiting until done, naturally.
The reason they don’t recommend using that probably is because this has a lot of overhead. Also it works only with threads that have a NSRunloop running. It’s really good for updating the UI from a secondary thread though.
For more heavy duty lifting you should use shared memory (with locks or lockless algorithms) for inter-thread-communication. Or even better use something like NSOperationQueue or Grand Central Dispatch and don’t worry about doing the communication and synchronization yourself, if your problem permits that.