mergesorts | mergesort in many languages | Translation library

If you're sorting an array that fits into cache, then quicksort will require fewer memory accesses just because mergesort needs to allocate a second array. Quicksort will load the array into cache and then proceed without waiting for memory at all. Mergesort will pay the additional cost of accessing the second array.

If you're sorting an array that doesn't fit into cache, then quicksort still wins from a locality point of view, because as they recurse to sort smaller sections, both algorithms will soon get to sections that do fit into cache, and for those quicksort is faster by the above argument. On the upper levels of the sort that don't fit into cache, quicksort and mergesort perform pretty much equivalently from a cache locality point of view.