I'm experimenting with my Hackingtosh running Cubase4 under OSX 10.4.9 and have found that turning off multiprocessing gives me better performance (less clicks/lower cpu hit in perfomance meter under lowered latency). Does this mean that only one of the cores in my core2duo is functioning?

Anyone else have the same problem?

I also use CB4 with the same setting (after reading a few posts on the Cubase.net forum) but I don't perceive it as a problem. The performance is definitely better and it seems everything is more stable, especially with lots of VSTi instances.
I'm not a tech head but AFAIK multiprocessing has nothing directly to do with the number of cores. Aren't there single cores with multiprocessing? Also it has nothing to do with running CB on a hack as this hint is actually meant for a 'real' Mac.
Please tech gurus, correct me if I talk rubbish.

Weird thing is that in XP, C4 works better with multiprocessing on

Now you're talking, mate.
I have MS products banned from my personal gear list so I've never tried C4 in XP. I would imagine (once again with little tech background) that the Windows architecture is so utterly different from OSX (and all other OSes) that the multiprocessing option was in fact developed for Windows only.

on my system playing low latency vst instrument. it works better when mulit is off indeed