It has been reported by some users that have very powerful computers that the game at times start to lag/perform poorly. Some solutions has been found as well.

If you are using a Nvidia card, go into the control panel and locate Threaded Optimization and set it from Auto to False.

Nvidias 93.71 driver release seems to cause some performance problems, try reverting back to an older version. For example 84.21. See related articles for instructions on how to properly install drivers.