Well the way a Step/Dir servo works is it takes Step/Dir signals and the drive outputs the voltage to the motor based on that. The Step/Dir is based on the encoder so if you have 10,000 pulses per rev on the encoder and you have 2:1 reduction and 5mm pitch screws then the steps per unit required in Mach would be...
10,000 x 2 /5 = 4,000
So even if your computer is good enough to get the kernel to 100KHz the max velocity you could achieve is 25mm/second or 1500mm/min.
In reality you will likely find 45KHz to be a more realistic kernel speed from the parallel port so your Max velocity would be 675mm/min.
Now there is a way to get round this, most servo drives have a form of electronic gearing where you can tell the drive that 1 pulse from Mach is equal to a certain number of pulses, so say you set it to 4 then that would allow you to achieve 4 x the 675mm/min, set it to 10 and you could get 10x the 675.
Downside of using electronic gearing is it can make the motion rough (cogging) at low speeds and that is where the fast pulsing of most external controllers comes in, it allows you to use Mach with no electronic gearing in the servo drive.
Hood