I'd stick with the early suggestion of drive motor. Temp. sensor. Motor grounds (look at voltage from motor to the truck frame). A dirty motor (brush dust) can cause problems.

The only rec. that could cause a slow travel speed is the recirculating (flywheel) rec. if shorted it will blow the power fuse. If open it would cause slow speed in scr range. the other rec. is for plugging. If open the plugging will be very harsh. if shorted I don't think the truck will run at all. If any of the scrs have failed the truck won't run either.