I have always low FPS from 20 to 35 maximum, with 60% CPU use in SP and 70% in MP, 35% of maximum GPU use in both.
Video settings can be changed with only 4-6 FPS difference from standard to ultra...
I tried to update video card drivers to latest beta and overclok the CPU without success..
Terrain and object distance really affect FPS, now I have it to 5000 terrain, 2000 obj (and 100 shadow)...