i used to argue with housemates over this constantlyyy!!! so funny to see it come up on cs.
my take is that i dont think it really matters IF you're cooking ready meals/frozen pizza/etc, but if you're cooking food from raw, i think it's gross to not preheat because the food isn't getting the proper amount of time to cook and undercooked meat is a great way to get sick.
it feels like im waiting longer overall if i don't wait for the oven the preheat, because i have to let the food cook longer to cover the improper temperature... whereas if i just wait for it to preheat, i don't feel like i've wasted time xD