Even though electrical resistance heating is 100% efficient at turning electrical energy into heat and other forms of energy that quickly become heat, it seems to be less efficient at heating food than microwave heating. The same dish can be cooked or reheated in a microwave oven about ten times faster than in an electrically powered conventional oven. Therefore, I think it might be beneficial to use microwave heating rather than resistance heating in the context of pressure cookers and slow cookers.
Pressure cookers usually already have metal pressure chambers, which would serve to contain the microwave energy. The only change necessary would be the material of the internal pot, which is likely to be metal, currently. Slow cookers generally also have metal bodies, but not metal lids, so those would need changing; on the other hand, they generally have ceramic pots which should be radiotransparent already.
An advantage of microwave heating, which may account for for its seemingly better food-heating efficiency, is that the heat is produced inside the food, whereas resistance heating has the heat produced in the heating device, from where it must be transferred to the food, but not transferred to the surroundings (because that would be a loss of heat and therefore a reduction in efficiency). Therefore, the thermal insulation is easier in the microwave heating case, too: it just has to be around the food container, not around the food container and the heating device.
A further advantage of microwave heating, at least in the slow-cooking context, is that it can turn on and off more quickly. Keeping the food warm over several hours should only require intermittent pulses of heat; a magnetron is more suited to providing those than a resistance heater, because the resistance heater has to heat up to a certain temperature before it can transfer heat to the food. (I'm assuming it cools down faster than the food does in between pulses, due to having less heat capacity and being closer to the insulation.)
N/A [2019-10-07]-- notexactly, Oct 10 2019random, halfbakery