Lowering the in-game settings doesn’t make a difference. That’s a telltale sign of a game that’s not optimized.
As for the hyperthreading question, I have a 4-core CPU overclocked to 4.5GHz (i5 3570K) with no hyperthreading, and I have much of the same problems. I’ll even see the framerate dip out in the field sometimes if there’s any sorts of effects present, like desert wind.