The speed of the painting for any given paint preset is a function of what is going on inside of the paint synthesizer controls. So how it's programmed basically.
By adjusting those controls, you can typically speed up painting for any given preset, although you may be trading that off for some other aspect of how it looks.
You can also design paint presets from scratch with the goal of restricting them to run fast.
One quick speedup edit is to go to the paint synthesizer's Path Application control panel in the Editor, and increase the Spacing parameter there. Increasing the spacing between adjacent paint nibs on a painted path will speed up raster paint presets.
Using vector paint rather than raster paint will dramatically speed up painting, since the vector painting happens on the GPU in your computer.
Clever paint synthesizer programming can also speed up painting. Depending on what kind of artistic effect you are trying to achieve.
Here's a tip on controlling where automatic paint strokes are drawn.
http://www.synthetik.com/tips/2010/01/controling-where-automatic-paint-strokes-are-drawn/