You need N joules to heat up the 300kg of the engine. It doesn't matter if you sit there for 15 minutes freezing your ass with an engine running at 1500rpm (and all other car parts still freezing) or 5 minutes actually driving at 3000rpm - you still spent N joules on heating, difference is the time. And the car heater runs on the engine heat, not on the electricity, so the sooner you would warm the engine the sooner your car interior start to warm and stops seeping the heat from engine.

It's actually more importantly to have enough viscosity in ATF, but again, the sooner you would go the sooner it would warm.