At least in terms of memory management. OSX is terrible at both starting new apps, and managing memory when switching between a lot of apps. At 8GB my macbook pro just could not cope with daily development work, continually running out of memory and spending minutes swapping bits in an out. It was unusable.
Even after my recent memory upgrade to 16GB, launching a new app on OSX can still take an age (for example opening a document that launches open office took about 30 seconds to load). Even quitting apps in OSX can be undesirably slow.
Yet on my windows desktop machine, with literally all sorts of crap running on it both in foreground and background, when I open a document that launches open office its there ready to use within 2 seconds. Photoshop loads in about 3 seconds. For comparison, Gimp on OSX took 30 seconds to show the splash screen and 50 seconds overall to fully load.
Microsoft beats Apple hands down here. Windows just feels much more responsive.