I had this same problem an hour ago, but restarting my computer fixed it. (I am playing DayZ for at least a month now, and this happened only today) This is what happened: I was playing in a night server (BRA2) and all of a sudden my FPS dropped from 70 to 5~15, i thought "oh fps bug, let me restart the game and choose other server". I noticed right away that the main menu was kinda laggy, and after i joined another server my fps was still around 10. Then I found this thread and decided to check Dota 2, and my fps was around 10 as well, and i normally have near 100. This all fixed after restarting my computer, but I still think there is something wrong with the newest ArmA 2 beta patch or DayZ...