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On my Mac Pro I have the same problems when using Safari or Firefox and viewing myspace friends with "pimped out" profiles, it becomes unstable and super slow. I boot camp over to windows and its perfect. Many sites just work better and faster on IE windows, but I usually avoid them which is their loss.
 
Flash on the Mac has ALWAYS been poor compared to that on a PC. It is very likely correct to blame Macromedia (adobe). Even back in OS 9, the difference was glaring. It is not based on your browser. It is based on the fact that Mac flash is a different app than the PC one, and obviously written poorly. It is particularly poor at scaling to large sizes. A flash file will speed up or slow down based on how big its dimensions are.
 
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