Revo loss of speed

I thought I would get a short run with my Revo tonight up and down the street and the speed really dropped down I am guessing in the neighborhood of 20mph @ WOT I am guessing its the slipper I put my foot at the front wheels with the throttle on and the pinion and spur is moving, but thats it. You can hear the motor winding out no problem, but its time for bed so I will check it out tommorrow