The main use of a Primary Domain Controller is authentication. That does not seem to be a task that requires much continuously in terms of CPU or RAM. Of course the less services the machine has to provide, the less it has to swap between tasks. If you run any graphical (runlevel 5) stuff ditch those first. At the Samba site there's probably more specs, but here's one
estimation of memory requirements, though for that example. If you think you experience problems with auth, the first thing would be to look at the logs (both sides). If you think you experience problems due to CPU/RAM/network bottlenecks, running Atop (-w), Dstat or SAR might help identify it/them.
