A Quick One on Quicksort

Quicksort is quick because it is memory efficient - O(n) with a leading coefficient of one and a small constant overhead (a couple of pointers. The memory efficiency reduces IO. IO is very slow and the reduction is what makes Quicksort quick.

Without accounting for memory efficiency and IO, other sorting algorithms...notably Mergesort are faster in theory.

Quicksort is quick by virtue of engineering rather than mathematical proof.