I wish I could help you optimize.. but I have a 800 mhz and I get similar problems only when I am working with large brushes on a large canvas.. One thing that has helped me is lets say I eventually want to work on a peice that is large... lets say 300 dpi for printing.... I will start out with a small peice around lets say 300pix x 500 pix.. and block it in.. then get a little detail.. then I will resize to 600x1000 and start adding details.. that way I dont have to use large brushes... and it never gets sluggish. Then once I get that where I want it and need to add even more detail.. I will then double it again and continue to go in and add more little details..
catch my drift?
Hopefully someone has some incite on how optimize... anyhow good luck
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