Same problem here. Development branch Hardly playable at night. Mostly noticeable in air station Stratis (but still very low FPS everywhere on the map). Sometimes game even "freezes" for few seconds (not a MP sync issue). I haven't noticed any performance issues during daytime.
os: updated Windows 7 64bit
cpu: Intel Core i5-2500K
gfx: nVidia GTX560Ti with driver version 314.22