My OS is Windows 8.1 x64, btw.
I found this but it's just to do with crashes, not performance.
A couple other things I've tried:
Setting Priority of the exe to "High" - no change
Setting Affinity to all cores and each of the four cores - no change
It's hovering around 15 FPS.. anti-aliasing, anisotropic filtering, resolution seem to be irrelevant to performance and the only in-game setting that changes my FPS is Shiny Water, which was identified as a common culprit years ago and everyone knows about it.
I think it's just something to do with the AMD driver not playing nice with how NWN implements OpenGL.