Terrible performance on gaming PC

Dec 8, 2018
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Hello. I bought a new gaming PC from my friend about a month ago. It´s decent PC in my opinion, however the performance is terrible. I tried 2 games already and both run horribly. First is Assassins Creed (2007), which runs perfect, but at places with smoke, bigger crowds and more complex enviroment, the game drops FPS drastically, lets say even to 25 FPS. It is on the highest settings/details, but this game is so easy to run, that even my calculator level laptop runs it pretty fine. Second game is Assassins Creed Brotherhood (2010), which is a lot worse... FPS are like 15 to 20, sometimes 25 to 30 and I just cant figure out WHY. PC was set in factory settings and reinstalled, before I got it. Of course, all drivers are installed (Scanning told me). I hope I did not forget anything. Sorry for english, Im Czech.
PLEASE, smart guys, help me I tried everything already.
My friend ran The Witcher 3 on the highest settings and I cannot run AC?
Here are my specs:

OS: Windows 10 (64 bit)
RAM: 8 GB
CPU: Intel Core i5-4460 3.2 GHz (3.4 GHz- booster)
GPU: AMD Radeon R9 270x 2 GB
I hope thats all...