I am wondering this myself.
I just found an old microwave oven in the basement. I decided to gut it and found that besides microwaves it also has a convection system with a 1400W heater in the back, and two IR elements on the top (650W each). I ripped the magnetron/transformer/cap and the rest of the electronics, only kept the two IR elements, the convection system and the light.
I did a fast test and the oven gets to 230 degrees C in 3 and a half minutes.
IR only heats the surface, not the air around it.