sorry for the somewhat cryptic answer I gave earlier. "Hot rodding your brushes" written by John Derry should go a long way to answering some of your questions.

In the meantime, play around with the "feature" slider visible when you select the Size line under the Stroke Designer tab in the Brush Creator (control-b)

If the brush preview at the bottom has a red "X" or doesn't look like a smooth curve, that particular brush will perform poorly on your machine. (I'm looking at a Painter X interface, so it may be different than yours.)

-pixel_streamer