UPDATE:
If I only use the pistol and do not select the cell splitter or the thunderlink, I am able to get to around 6:30 minutes and around 250 kills before the wave that spawns basically stops my frame rate completely. The big monster that spawns from the cell splitter seems to have a huge impact on system resources as well as the later waves. I am actually upgrading my system this week so I will update this on how it affects nighmare's stability on the client side.