I bet the problem is somewhere else to find. As if some bottleneck is screwing the whole thing up.
I ran Nexuiz on at least 10 boxes from 1.2GHz systems with Nvidia GF2MX up to several dual cores with GeForce 8600 and ATI HD2600XT, and there's only one time I remember I had problems. I think this was my debut hree at the forums.
The only time I experienced issues like you do, I just had the wrong chipset drivers. Nexuiz was the only game I had problems with at that time. Are you sure your systems are all up to date and running fine?