The slow turning of the engine may be the results of the friction drag at the cylinder walls/pistons. I will suggest bench testing the starter. Jump it with a 6 or 12 volt battery. pos to the switch lug, neg to the starter body. the short test should have the starter turning with high RPM's. Any friction drag with-in the engine will cause the 'too slow' condition. With the plugs removed, try turning over the engine.
If the commutator is dirty, it may be cleaned with a piece of wet/dry 400 sandpaper. However, if the commutator is rough, pitted, or out-of-round or if the insulation between the commutator bars is high,this condition suggest a rebuilt starter may resolve your problem.
Good luck
