When you cook something chemical reactions are occurring. So essentially there is something called the activation energy for a reaction which is a certain energy amount that the particles must collide with (while having the correct collisional orientation), and if that is not reached then a reaction between molecules won't occur.
However, if we heat it like we do during cooking, we are providing a source of kinetic energy through the conversion of heat to kinetic energy for the molecules and this means that more of them will be able to overcome that activation energy barrier due to a greater average kinetic energy
So the difference between lower and higher heat is, it takes longer for the heat to be converted into kinetic energy for lower heat