It me takes longer to install a large Mac program from the .dmg than it takes to download it in the first place. My internet connection is fairly slow and my disk is an SSD. The only hypothesis that makes sense to me is that MacOS is still riddled with O[n] or even O[n^2] algorithms that have never been improved and this incompetence has been made less visible by ever-faster hardware.

A piece of evidence supporting this hypothesis: rsync (a program written by people who know their craft) on MacOS does essentially the same job as Time Machine, but the former is orders of magnitude faster than the latter.