So I recently upgraded from the Intel HD Graphics to a GTX 750 Ti, I ran the game on Ultra @ 1080p and it was smooth then the FPS dropped down, firstly I thought it was a Fraps shit so I closed it and lowered the graphics to low, still same lag. After a PC restart I played the first mission on Ultra @ 1080p again, smooth as fuck. Graphics were astonishing, but then at the second mission it started to lag again even on low, weird shit is weird. I guess it's a CPU bottleneck (Core i3 2120 @ 3.3GHz)