@Enzoconti, thanks for the reply. @Cre8thenew, I'm not working on a network, but on an (external) eSata disk. What's even more strange is that my late 2008 Macbook Pro (4gb ram, 2.54Ghz DuoCore) is running ID CS5 more smoothly. Still slower that CS3, but better than CS5 on the Mac Pro!! I really think it's an issue with the ATI gfx card in the Mac Pro. Sometimes when scrolling on de Mac Pro the whole document turns white. Scrolling again get's the screen redraw.
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