If the PC can handle the step generation, then it should handle threading, as IIRC threading is handled in the low level parallel port driver code.

I used to cut threads using my old Conect with it's DC spindle motor, although it was running a Smoothstepper, but it was still only using a single slot sensor. I'd guess the toothed belt drive from the motor probably helped by eliminating belt slip and reducing how much the spindle speed changed under load, as the KBIC done a good job at maintaining speed under load.

It does seem to be something that works perfectly well, or gives endless headaches.