My daughter has a 3 car attached garage with one of those hot dog type heaters. Opening the door for a short time and then closing it doesn't seem to have too much effect. The problem with heating up from 40 to 55 or so that will take a while is that the floor and walls and the contents are all cold and take a while to get up to temp. so the air might be warm but heater will need to work harder because of all the cold stuff in there.
If I did the calculation correctly, heating the air takes about 200 btu per degree or 3000 btu for 15 degrees F. If your furnace puts out 60k (75 k input) that is 5% of an hour's worth or a couple minutes.
Seems like not much, but it doesn't take much energy to heat 9000 cubic feet of air. Please feel free to check my numbers, since I did the arithmetic in my head and I could easily have misplaced a decimal point.