I replied earlier in the post that I don't have this problem in Fallout 3, and I'm running Win7 x64 with a GTX 470. I don't notice the problem with NV until two or more NPC's are in my field of vision, then I notice a slowdown. Using the "toggleemotions" command in the console clears it up. I finally gave the .dll a try, and it seems to work too. I might stick with the .dll for now because you have to type the console command every time you start the game, and I sometimes forget until I see the slowdown.

I'm running FNV on vista 32 and I'm also getting the NPC lag, although it is not as bad as some others are reporting.