Thanks for quick answers and suggestions but I have already fixed my problem. The problem was, that the laptop was overheating and when the temperatures were near 100'C, CPU was slowing down in order not to overheat. Bought this cooling fan
and the problem went away. I was ignoring this issue in other games such as F1 2011 and FIFA 12 but my games started laging after some time of play because of the temperatures. For the fellow Acer Aspire 5742G users (if there are any reading this post) I want to advise to switch off TurboBoost (10x to OverTheBelow have a
) and to buy a laptop cooler in order not to damage hardware on long gaming sessions.