Anything below a stable 40 fps is not playable. I've seen many people making that error (including my own beloved main admin). 10 - 20 fps is really crasher level type of racing.
With < 40 fps you are missing every apex and crash into other racers during close racing.
It's partly due to an overloaded system, delaying the input it gets from the human interface device (mouse/gamepad/wheel.) and partly your eyes receiving not enough information to act on time. Its also really exhausting to drive and can cause headaches or other serious problems. (not kidding!)
Actually it would help a lot if Scawen would made a warning if it detects a low frame rate.