Thanks for checking out the game!
The game is, indeed, pretty physics-heavy. Could you open a hardware monitor on your second monitor and check if you are getting hit by the GPU or CPU usage? There is also a graphic detail slider in game that might help.
A good simple way to check is to kick in the adrenaline booster. If your framerate increases, you are most likely CPU-bound. I can offer more advice once I know these.