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?