starter questions

The older large OEM starters had bushings at each end of the armature.
With age, the bushings wear, allowing the armature to be sucked over by magnetism of the field windings, and literally " drag" the armature over the fields, slowing the speed down, the more the bushes are worn, with heat and expansion, the problem increases. When cold, the drag is near non existent.
You could always tell by the shiny areas on the armature, and fields, - as "witness" marks .
A few dollars spent replacing the bushes, and electrical "contacts" would remedy the prob.