My PC has GTX980m, 16GB ram, WINDOWS 10 professional edition and I7-6820HK, and I have turned the effects downf my game into the lowest rate. In normal, it does has 50-80FPS, however the FPS drops into less than 10 nearly every half minnute, and I cannot ride horse or even walk in a normal way because of the lag.
I found that though my PC has 16GB of RAM, in Windows taskmanager no more than 50% RAM was used when this game running. I wonder if it has relationship with this problem.
I bought and download this game from Steam. And I have palyed a bunch of video games without this problem.