Andy has put his finger on it - the issue is not that Mach3 uses significant cpu power, but that it needs to be able to issue step signals in real time with as little delay as possible. Any other activity can cause small delays, which translates into uneven signals sent to the drivers, which means rough running. That's why laptops, for example, aren't supported by Mach3 via the parallel port - the cpu has a habit of putting user jobs on hold while it does some internal housekeeping which wrecks the steady pulse stream needed. If you really need to run two things at once, best thing would be to add a motion controller on the Mach3 side and scrap the parallel port. These buffer moves so that slight cpu delays aren't significant. However, I'm a little concerned that you also talk about Mach3 using the serial port. That's not usual, so I wonder if there is some other complication in your setup.