Peak times (late afternoon/evening US) especially on weekends have become a lot laggier over the last several months. The lag increase has been direction proportional to the number of people we have playing at those times.
CNR indirectly causes lag spikes when people log in (as L said) since it creates a lot of items and gives people a reason to carry around a lot of them.
However, I dont think it adds to the "overall" lag you experience on average during a time interval.
I think that sort of lag is mainly casued by (and determined by) the number of people in the most crowded single area on the server. This is due to the way the NWN engine handles multiple PCs in one area.
Thats why the lag is worse when a quest is being run even if the overall number of PCs on the server doesnt increase. If you have 30 PC's on the server spread between 20 areas with the most crowded area only having 3 PCs in it the amount of lag you'll experience overall (barring possible login spikes or other oddities) will be quite low. Now say a quest starts, no other PCs join the server, but 24 of those 30 PCs all go to one area. The lag on the server at that point becomes what I would describe as "reaally bad."
-TV