well, not sure what hardware you have, but if your CPU is too slow, or you don't have enough VRAM that can cause big drops in a game.
what do you have?
Windows version (i.e. W10 64-bit)
CPU (i.e FX-8350)
Graphics Card (i.e. GTX770 2GB)
System Memory (i.e. DDR3, 8GB)