Because it is electric heat, it is pretty much immediate and heats up quick. Out winter (like yours) last year was milder than normal, so I mostly went with the heated seats over the heat. It was my only winter I had the car. Since I will be working from home at least until Feb 21, there will not be much winter driving this year either.
Maybe I will be much more rich by winter 2021-2022, so that heating and lowering my range will be an afterthought.
Yeah, I don't know why the Prius Prime's heat was so slow. The seat warms up similar to other car's seats because all seat heating are electric, but the air from the outlets just won't warm up after quite a while. Good to hear that Model 3's heat is near instant. Will definitely need to try it this winter. NJ winter gets pretty cold at times.