I'm have the same problem 15 or so fps in cities and 25-30 fps in the forest also consistent 1 sec or so freezing like lag spikes. This happens also in the mod but not any other games like bf4 that I can run ultra and get 60fps 99% of the time.
system specs-
cpu - amd fx 8350 @4.5ghz
Gpu- evga gtx 760 sc 2gb
ram- corsair vengence 8gb
ssd- samsung 840 120gb
HDD- 1tb western ditgial caviar black
case- phantom 410
psu- corsair cx600
mother board - gigabyte fx990ga ud3
cpu cooler- cooler master siedon 120m