Brushed is pretty linear as far as load-->temp goes. Less load = lower temp, higher load = higher temp. So if you gear faster it will get hotter. Timing advance in brushed motors raises the kV rating which is just like gearing up, so it will get hotter then, too. The magnets in those motors are a ceramic material which (I think?) is
less prone to magnetic loss at high temps.
*edit* Found this handy info
http://www.magnetsales.com/Design/Matlprop.htm