Could be a number of things, so start with the easiest first and then its a process of elimination.
If the motor is turning slowly I would be looking at possible low power from a poor battery (check it properly with a load tester) . Other possibilities could include worn motor brushes , maybe even the magnets have come loose from the motor body. If the motor turns freely with no grinding noises when you turn it by hand you can eliminate a few of these possibilities - but it might be worth taking the motor apart to have a look and see if everything is ok.