Due to the complexity of running a paint simulation that tracks more than just colour, you will find that ArtRage runs slower than some other apps. Basically, the application is doing anywhere up to five times as much work as simple colour paint apps and we're really pushing the capabilities of the device. It's the price we're paying for doing physical paint calculations. If you're doing rapid sketch strokes then you may find some of the strokes drop due to processing speed and the time it takes to store strokes in the Undo system after you complete them.
That said, it shouldn't be so slow that it can't be used - Is it possible that some of the problem is actually the Stroke Gap (explained in the help system in the app under the memory & speed section)? The gap between the visible end of the stroke and the input point is due to the way paint blending works and you should find that that gap closes instantly when you lift your finger.