freevec.org

Assuming you refer to the memory functions, it's because data has to be fetched from the actual memory rather than the L1 or L2 caches. And AltiVec has an 128-bit bus but to the L1 and L2 caches not to the main memory. Still, I use cache prefetching in most of the functions and the performance will still be better than the original functions. Don't expect miracles though, in these cases a 20-30% performance gain is more likely rather than a 10x.