I've been having hideously slow performance in Photoshop CS4 when using the brush and other features. After unsuccessfully trying to Google my way through my problem and trying out what worked for other people I stumbled across my solution. Someone suggested turning off OpenGL in Photoshop CS4.
Instead of completely turning off OpenGL I first tried just turning off vertical sync and voila! Silky performance as smooth as a newborn's butt!! To get there simply go under Edit > Performance > Click on the OpenGL options. In there I only turned off Vertical Sync which was enabled by default. The difference is night and day, its buttery smooth now.
Now every computer is different but its worth a try if you are having poor performance. You can also try totally turning off OpenGL also.
Hope this will turnout useful for someone out there.
Oh btw my computer is a Dell Studio XPS 16, Intel Core2Duo 2.66ghz, 4GB DDR3, ATI Radeon Mobile 3670 512MB, etc.
I'm on an amd X2 4600, with an 8800gtx and 2gb ram, running open GL and I havent got any problems.