There are a few reason that are likely.
#1 You are using gnome but backtrack is designed to use KDE so you actually have twice as many programs as you need (2 complete Desktops)
#2 ATI support is extremely poor. Most likely the freeze probelms are due to the Xserver crashing.
#3 You are using a old IDE hard drive. You make a comparision to mac on sata vs. Backtrack on IDE.
Backtrack and Linux in general is very efficient . I would put my Gentoo linux box up against any Lepard OS any day ( and win)