With all the references to thermal conductivity and whatnot - nobody's mentioned the importance of thermal mass yet. The stove doesn't have to just heat up the cooking utensil - it has to cook the food too. Drop food into a thin-walled cooking utensil and the heat is immediately absorbed by the food - and you get to start from zero.

Drop food into a cast iron utensil and the thermal mass will start the food cooking immediately - and continue after the stove has been shut off. Total calories required to cook the FOOD doesn't change.

Thats pretty basic stuff that was taught in high school.