starter questions

I have an 09 Charger Police 5.7L with a starter issue. When cold, when you hit the key, the starter cranks normally and the engine fires off immediately. But when fully hot, if you step away from the car for 15mins, and go to restart it, the starter will crank really, really slow. Surprisingly, it will still start the engine (so far), but I think it's only a matter of time when it will fail to start. My question is, what makes a starter do this? If I take it apart where should I focus my attention?

The most common cause of a slow hot crank is a bad starter. As the brushes and the armature in the starter wear they begin to bind then the starter is hot causing this slow start. It can also be from a failing positive battery cable that has a lot of internal resistance.

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.

^^This right here^^

On a PM starter I don't know, loose magnet maybe, or same problem, worn bushings.