If, and that is a big if all the plastic gears are intact there is a few reasons for the hard turning.
The machine needs oil. There aren't as many places to oil, but there are places. The main shaft where it passes through the arm and especially under the face plate behind the stamped light reflector. Under the bed there is also places to oil.
Our 4622A, similar to yours, had some icky white grease on the plastic gears. It had solidified. I cleaned them off and regreased them with Tri-Flow grease. That made a noticeable improvement.
So I suggest Tri-Flow oil and grease.
Oh, also check the motor. Ours needed the commutator and brushes cleaned and the felt pads on the ends of the armature shaft oiled. Another major improvement. For the motor I use regular sewing machine oil.
Joe