Slow is comparative. When you say Imac, because you do not give its specs, I automatically think 'Slow' for starters.

I'm running Painter 11 on an Intel 9850 2.8 ghz Quad with 8 gigs ram, an n'Vidia 8600Tgraphics card, fast Sata 2 drives on Win 7 Ultimate 64 bit. Painter 11 is fast, never gives brush lag, and has quite fast I/O operations.

Painter 11 is a power hungry application, and very picky about machine configurations, especially with respect to graphics cards. I have another identical win 7 machine--Except that it has a motherboard with onboard graphics--that I cannot get either Painter or Essentials 4 to
work on. They install but the applications get just past the initial splash screen , the graphics are all distorted, and loading freezes.

If I had to have Painter on that machine I'd have throw in a good graphics card and disable the onboard graphics.

Configuration compatibility is paramount with high end programs.

Maybe if you give your Imac specs, some detail about how it is slow, someone with a similar configuration may be able to help.