Hi,
It seems like you are already tweaking in the right place. Sadly the game engine is a bit heavy on performance
Have you tried playing in windowed mode with lower resolutions ?
Shadows and particles generally have the most impact.
Depending on your hardware, vertical sync may also have an impact, you can try with and without, fullscreen and windowed.
Unfortunately we can't do much more to improve the performances without changing the assets to a lower quality.