depends on target temperature, and how much gradient from internal to external temperature is acceptable. high heat will cook the outside much faster than the inside, which is fine for something like rare steak with a seared outside and relatively low core temp, but cooking chicken the same way will have the outside completely dry and charred by the time the core gets where it needs to be. lower heat gives more time for heat to spread all the way through, so the difference between core temp and surface temp is less