First of all, are you using the newest version of RoR? (4.8.0 RC5)
If you are on Windows, make sure you're using DirectX9 as the renderer, in my case there's a major performance improvement over OpenGL
If your computer has a graphics card and integrated graphics, you can make sure that RoR is using your graphics card rather than the integrated graphics.
Other than that, I can recommend messing about with the options in the "Graphics" tab in the settings, maybe turning off realtime reflections, changing the sky and water type and just trying and seeing what setting or combination of settings makes the biggest difference.
What car/map are you using? some of them are more resource-intensive than others, from what I've noticed
Hope that helps