I was just about to make a post regarding this, but looks like I won't have to. Back on my old PC running WinXP I never experienced lag with NwN, it was only after I upgraded to a PC with WIn7 (and, as I understand it, the game has some issues - framerate being one - on Win7).

Kinda hoped the GoG version might've fixed this, made the game more compatible, etc. Guess not, then. :x Oh well, just means I won't be buying a third copy of Neverwinter! xD