Apple Mail vs Thunderbird performance
usualy mail applications all lay around the same in preformance and memory depending on size of inbox, file sizes, nr of RSS's there are many variables, but the general thing is they only use enough to have an impact on performance, whenever they are grabbing new information! (not even then) but in order to minimize CPU usage you should set "grab mail" too manual

as far as tip on good mail client.. check out sparrow!