Game performance. (The Red Solstice 2: Survivors)

Who is like with FPS in the game? I have an i7 4820k, 16 gig DDR3, GTX1078. In the menu and on the card, it is stable at 60fps, and on a task from 20 to 50 it jumps, and at any graphics settings. It seems that the optimization of the game is rather weak. My computer, of course, is far from new, but it shouldn't show 20fps in such a game, as it seems to me.