In a keynote speech, Sony Santa Monica explained that the SPU processors aren't actually co-processors but are full general purpose processors that can handle code directly lifted from the primary processing unit. According to Sony they are not just fast but are "devastatingly fast."

How fast, Mr. Wolf?

Well, it might interest you to know that the God of War II dev team rely heavily on the SPUs to accelerate both the PlayStation 3's graphical power and chip (the RSX) and the PPU processors. They then detailed the systems running on the SPUs, which include animation, cloth, collision, procedural textures, culling, shadows, push buffer generation, meta tasks, geometry conditioning, and sound. Indeed, most (if not all) of the heavy lifting is left to the SPUs.

Think of it in desktop terms. You may get decent graphical performance from an integrated board, but if you have one that is dedicated, with its own RAM and not a part of your motherboard, the results can be astounding to say the least.

The team concluded their talk by reiterating that developers must use the parallel nature of the PlayStation 3 and not treat SPUs as a special case, since they are much faster than you think they are.

Source: MaxConsole