The Mac runs at a lower clock speed to the PC. Contrary to popular opinion, clock speed is not directly linked to actually performance.
Just because a processor can do 2 billion cycles per second doesn't mean it actually gets a lot done. This is the whole philosophy behind the processors Apple use. They get a lot more done per clock cycle versus AMD/Intel chips. The clock speed is only a guide to performance when the chip architecture doesn't change. This makes all comparisons between the performance of Apple and x86 chips on clock speed alone useless.
If you still don't get it, think of it this way. I build a chip that take 10 clock cycles to perform 1 calculation. I run it at 50Hz, so it can do 5 calculations per second. You build a chip that takes 5 clock cycles to perform 1 calculation. You run it at 25Hz - while my chip "runs faster" than yours, they still both do the same amount of "work" each second they're running.
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