Now that the GUI has an FPS display, I was able to try the different options last night and see what made a difference to performance. The answer is: not a lot.
I switched every option down from high to low, and only one made more than 1-2 fps difference: sampling. Supersampling halved the frame rate, dropping from 15-16 down to 8. Subsampling (rendering at half resolution and scaling up into a blurry image) increased it by about a third.
So I’m not quite sure where the bottleneck is. Clearly the GPU is limiting the game to some extent because rendering to more pixels slows it a lot and rendering to less speeds it up somewhat, but the main limit seems to be CPU. Reducing LOD distance to low made little difference so it doesn’t seem to be graphics geometry.
Interestingly, I noticed last night that my Norn character is getting 20-25fps whereas the human gets 15-20. There must be something different about the two zones.