I feel your pain on the low FPS. In Alpha I had great smooth FPS. Now most servers give me around 20-25 FPS. I have to find those 'High FPS" servers to play at all. As for single player some missions aren't a problem, but others lag like hell.
My specs:
CPU: Intel core i5 2500K 3.3ghz
RAM: 8gb DDR3 1600
GPU: ATI Radeon HD 4850 (I know this is where my problem most likely is) I plan on getting a GTX 650Ti soon as I believe that should be enough.
I just feel like Beta is kind of more messed up than Alpha was. I mean the same problems exist with more added to it.