Latency of parallel port system on decent hardware.

Do you get latency warnings when running LinuxCNC?
If not, do not change anything.
If yes, download the Wheezy ISO and boot from USB, start your config by right clicking on the .ini file, open with, type linuxcnc and press enter.

For the lowest latency, you might have better luck with RTAI than PREEMPT_RT, assuming the BIOS isn't buggy:

forum.linuxcnc.org/9-installing-linuxcnc...ng-rtai-debs-for-2-9

If you need help, let us know!