S!
Unfortunately there is not much information on the settings in the conf.ini at the moment. It would be a great thing to have a guide of it what each setting means for the user, at least the ones that like to tinker. At least I do
But as a rule of thumb disable: shadows, grass, SSAO. Turn down to medium: lighting, texture size. You could also try the default Medium settings and then tweak the rest to your liking. But biggest FPS eaters are shadows, landscape textures/detail/lighting, amount of buildings and their respective settings + rear view mirrors in Hurricane and Spitfire. Those alone off (press "m") can give an extra 5-15fps!
I hope this helped.