I can't see a problem with parallel port timing as the mini-itx running Mach 3 will be handling all that. You'll just be using RDP/VNC for the display and input.
At worst, your DRO display in Mach 3 may lag a little.
I've been doing this just recently. The machine with Mach 3 installed is upstairs and to avoid the climb, I've been running VNC from a laptop. Note that this is just for getting familiar with Mach 3, no cutting attempted as yet. In fact, no cnc router is connected as yet, just a controller via the parallel port.
FWIW, my Mach 3 computer is a pentium 4 with 1 GB memory and inbuilt Intel graphics - so far, so good.