you know memcpy is my 'classical' example of why you should never trust library code to be well optimised. its almost always a dumbass byte-by-byte copy - its pretty much the worst case performance unless you go out of your way to slow down your implementation.

the speed improvement from good register usage and appropriate cache hints was >35% the last time i had to do this (iirc against the VS 2008 implementation)

MS provided a special memcpy variant for 360 that did this for you using the PPC dcb* instructions and good use of the wider registers.