I'll take a shot at this...
It may not be the processor that's the important thing: if your macro is doing a lot of disk reading/writing then it'll be the disk; if you're doing a lot of screen/image detection it could be the graphics processor; if you're doing network/Internet stuff it might be your network/wifi interface; heck - it could be your Internet provider!
If, on the other hand, you macro is doing nothing but calculations/text manipulation then it might be the processor.
And that's the definitive answer 
Oh - I forgot to mention RAM.
Oh - multiprocessors - see