Might be time to pull the starter. Look for worn bushings and rub spots on the field coils. Good brushes and a clean armature will help as well. You are talking about a very high current system with low resistance and any cruddy contacts or extra drag will not be helpful. That 12 volt battery should have spun the starter like it was on steroids. You have done the right thing with the cables so now it is time to look at the starter. There is a copper contact that can wear under the switch cover. I would look at that as well. The switch is available from lots of vendors including NAPA the bushings and brushes are as well. The starter is simple to take out, and apart so not to worry. Use big rubber bands to hold the brushes away from the armature when you pull it apart and put it together and it should be not too tough to fix.

Steve H