I had absolutely awful FPS when I first started playing it. Just turning around was a slide-show. I have a system that far exceeds the specs so I was quite surprised. (Although the installer only seemed to detect 2GB of my 6GB of ram!!

)
I stuck with it for a bit and by the time I was heading back to base on the tutorial mission thingy it was running smooth. Every time I have played it since, including re-doing the tutorial it has been fine. For some reason that first 15 min ran like sh*t.