It is a bit of a learning curve, seeing as though there is so much room to work- but there are 2 drive belts and a dual pulley called a variator which will likely need attention. The pulley has a movable center that has to move freely. Also, be sure that the brake is not dragging severely. If you replace 1 drive belt, replace them both.

I have an RX 75. Your going slow forward cuz your primary belt is gettin stretched. Just replace it and you be riding normal