[Al Skierkiewicz]

De,
I know many people will not look for an electrical cause for under speed motors, but I do. If you are using long wire runs, lots of connectors, loose terminals and under size wire, then you may not be getting full power to the motors. All of these things add up. remember that at the stall current on the CIM motors, even #10 wire will drop 0.1 volt/foot. Double that for #12 and if you have chosen to feed the CIM motors with 30 amp breakers so you could use #14, then quadruple the #10 loss. Loose connections can account for up to 5-10 feet of #10/connection. The same for bad crimps. Multiply that for loose connections on the #6 primary wiring.
Finally, have you calibrated the speed controllers in some way? The LED on the controller needs to come on solid when you are at the extreme of the joystick, Red for one direction and Green for the other.