I was thinking either the circuit supplying the slower motor is undersized or there is an issue with the belt or spool binding. It could also be high impedance connections due to corrosion, salt, grime, pitted contacts, etc. I would also time it, is it really slower or just sounds slower?
If it is slower with the same motor the only two things I could think of is the motor is producing less power or there are losses in the mechanical portion. For the former, it's almost always less voltage.
Sometimes Scotties just get a gremlin and once they get it they are unlikely to ever leave.