Before it can heat the food, a conventional gas or electric oven first has to heat some two to four cubic feet of air (“preheating the oven”), after which the hot air must transfer its heat energy into the food.
These are very slow and very inefficient processes.
A microwave oven, on the other hand, heats the food, and only the food, by depositing its energy directly into it with no intermediary such as air or water (as in boiling) involved.
The statement found in several microwave cookbooks to the effect that microwaves cook food so quickly “because they are so tiny, they travel quickly” is nonsense. All electromagnetic waves travel at the speed of light, no matter what their wavelength. And the “micro” in microwave doesn’t mean “tiny.”
They were named “microwaves” because they are essentially ultra-short radio waves.