I'd like to have the option to disable the 3d characteristics of the paint and paper, which I guess could result in there being only 24 or 32 bits per pixel that the Artrage engine has to take care of.
That would make painting in ArtRage very fast (CPU-wise). I know about paint thinners and no-grain paper, but that's different I think.
It's not just about speed, though. I mostly prefer to do my paintings in this way (flat paint); in Painter, i never use the impasto effect, and I just use a few brushes to create paintings with smoothly blended colors. (I do prefer the ArtRage interface over just about all painting programs I've seen, however).
(And I'd also like to have the much-requested ink tool and blender! )