I have a 2008 Mac Pro with the ATI Radeon HD 2600. Mountain Lion is obviously slow on it due to graphical problems - the card is really noisy when running Mountain Lion (or Mavericks), even if it's idle. All of the graphics are choppy most of the time. And yes, I checked Activity Monitor for anything else going on. It's definitely the GPU. I have no problems in Snow Leopard. It's listed as a Mac that should support Mountain Lion... I want to get this sorted out: Does this happen for everyone? I've heard a few complaints but nobody saying that it DOES work. Is there a software fix to this? Would upgrading to an ATI 5770 fix it for sure? I've heard that it can sometimes be fast once it boots then get slower. It's slow immediately. I've also heard that replacing 3 of the kext files with the Snow Leopard ones helps, but that either didn't help or made it worse.