Welcome to the forums! Nice to see your work.
Regarding speed, ArtRage performs well on my home machine which is of similar specifications to the ones you posted ( slower processor, more RAM ) If you're doing a very large image in terms of dimensions, things can certainly slow down as in order to simulate real media we need to store a lot of information in memory ( wetness, paint thickness and so on ).
Also, sometimes people can interpret the way that strokes end when you raise the stylus as slowness. The stylus will not place down the last part of the media until the stroke is ended by lifting the stylus, so it looks as if it's "catching up" but in reality, this is not the case. During a stroke we have to stay a little behind the cursor because we don't know which direction you're going in next before you move and we want to allow people's strokes to be as smooth as possible.
Memory management is something that we're optimising at the moment. However this will only tend to affect the performance when painting on large multiple layer documents in the full version.
Feel free to email me at firstname.lastname@example.org if you can use some support on this, always happy to see what I can do to help!
Resident Bug-Hunter and Support Guy