Look for a function in the transmitter menu called dual rates (or similar). It allows you to set two values that you can toggle between which can be used for reduced speed. So say with the switch off, you get 100% speed, then with the switch on, it limits the speed to 50% (as an example, this is adjustable). You can usually set different rates for different channels, I find it useful to reduce the left/right control more than the fwd/back control as it gives less erratic control when turning but still allows good straight line speed.