NWN doesn't support multiple processors and that manifests itself as unplayable lag. You have to set NWN's affinity so that it only wants to run on a single core.
Quick temporary solution Edit
If you are already running NWN, then hit Ctrl+Alt+Delete to pull up the task manager. Then, on that task manager, select NWN and there should be something labeled 'affinity' or something like that. Once you hit that, there should be a few boxes that are checked and you should uncheck one of them to get it working.
You have to do this every time you start the game, and that is not very convenient.
Permanent solution Edit
Set a processor affinity in the configuration file.
To fix it permanently, open your nwnplayer.ini file (it is located in same directory as the game) and in [Game Options] section add this line (or change existing one to =1):
- Client CPU Affinity=1
Now the NWN game will always use your second core.
source : patch note 1.67 : Added a new option to the nwnplayer.ini file under [Game Options] "Client CPU Affinity" (for Windows only). If you have more than one CPU on your PC, then you can use this setting to make the game run on only one of the CPUs by specifying the number of the CPU (0-31). Setting "Client CPU Affinity=-1" will turn off CPU affinity.