Having migrated from Skeinforge 40 to SFACT and print print a couple of small calibration objects things were looking up for SFACT. The first printed object started okay, but as it came time to lay out the exterior shells of some small diameter holes Mach 3 began to stutter.
Initially it was thought the problem stemmed from an overly detailed STL file, which did help with the symptom, but not on every print. In order to keep the buffer depth at a reasonable level print speeds were dialed back to 25 mm/s and even then there were problematic spots. Which is real disappointing given it was printing smoothly before at 70 mm/s with out breaking a sweat.
As it turns out, the motion buffering algorithm in Mach3 gets a little hung up if there is an F parameter in a G1 instruction. Comparing old SF40 to SFACT g-code, the old SF40 only set the the velocity every time it changed. SFACT, and SF40, will add the speed to every line by default. Turning on 'small g-code' in the export plugin removes the redundant F parameters.
With a queue depth hovering around 1000 and building almost instantly, things are back to their old screaming selves. So keep your g-code small!