Hard Starting Hot Big Block

What I can tell you about the old starter cranking slow hot, is the armature bushing wear out and let the armature drag on the fields.
They work great cold but hot, things expand and armature drags, the hotter or more magnetism/boost, just makes it drag more.
If it's a big starter, break it down, armature will have shiny witness marks on armature.
Bushing are just a few bucks,.
Check the electrical " stud and contact " condition
Good luck.