My computer is an HP Pavilion desktop w/4 GB RAM and Nvidia 8400. I'm running VISTA Ultimate 64 bit.
When I run Photoshop CS4 and have OpenGL turned on the screen painting and rendering slows down about ten fold or more.
Why would this be?
That's the question you and about a thousand others including me are asking, sadly
Adobe changed Photoshop's core with CS4 to use your GPU and it has led to major performance issues for many users and major performance benefits for others and no-one seems to know what the cause is or how to fix it definatively
Try this Google search for more info:
"photoshop cs4" slow OR poor performance