Erm. Sorry but this game has fps lag in 640x480 with very low graphics on a 750 Ti. I don't know what's the problem but I really don't want to find out.
Thank you for reporting issue. We will look into it. Our low spec testing machine was ryzen 3 with integrated gpu. It worked well at 1920x1080 and 50% internal scale.
Maybe internal scale would help in your case as well.
You can further reduce load, if you disable rear view and radar.
To eliminate CPU overload you can reduce amount of AI opponents in game config. (MayhemCars2_Data/settings.json)