Back in the old days, CPU and RAM were the same speed. Very quickly, RAM speed was left behind while CPU speed went through the roof. This is what we call "suckage" because while the CPU does it's thing, it's waiting on your RAM chips to send it poop. This is why faster RAM is nice and has nothing to do with the rest of this explanation. Don't ask me why I put it here.
CPUs aren't just the blocks you see them as. There's stuff inside them, mind you, it's too small for your eye to see, but there is stuff in there. CPUs are just like televisions, they may look the same on the outside but it's the difference between being able to get HBO and watch your pornos or having the thing go dead. Ok, they aren't similiar at all but there's more to a CPU than it's clock speed. There's a thing called a CPU's IPC, or instructions per clock [cycle]. It can even vary from clock to clock depending on what it's doing. This isn't a realistic example but let's say you have a CPU running at 200MHz with a constant, never changing (this never happens in the real world) IPC of 4 and another CPU at 800MHz with an IPC of 1. 200 * 4 = 800 * 1. That extra 600MHz didn't do poop, did it?