I just bought NV to process my old VHS tapes but when using it in Virtual Dub on my quad core CPU its sitting at around 55% utilisation across the processors.
The utilization may depend on NV filter settings (for example, temporal filter radius), clip size, VirtualDub itself and other VD filters used with NV as well as codecs utilized. A typical utilization figure in our tests on a quad-core computer is 85%.
As I said, the utilization depends on many parameters. Since I don't know what you use, I cannot give any specific advise. You may want to try adjusting the abovementioned parameters to see what makes a difference.