...and to put some numbers on that, at 750mm/min, Mach3 will be driving that parallel port at, maybe, 2000 pulses per second. 10 msec delay while the cpu is doing something for the other job means that 20 pulses will be delayed or maybe even lost. At best that means rough running as the steppers are continually changing speed due to varying pulse rates and at worst missed steps and lost accuracy.

You originally asked for people with experience to respond. I suspect that you might have trouble finding anyone who runs like this because it's such a potentially dangerous way to go. You are even recommended to avoid things like internet browsing while running Mach3 for these reasons. Use a motion controller like a UC100 and you might be in with a chance as the device unloads a lot of the time-critical pulse generation from the PC.