People should use the Search function more often. Just type in "FPS", "Lag" or even both and you should get at least 200 results explaining the same thing I am going to explain now.
The Game Engine does not use GPU Resources (even if it does, maybe just between 1-2%) and it relies on the CPU Performance instead, so a great Graphic Card wont increase your Frames per Second very much, yet.
The Developer is aware of this and currently in the process of re-coding the Engine to increase the overall FPS and add GPU Support. The coding process takes some time and there have been other Bug Fix requests that slowed the process down. The issues should be fixed relatively soon.