I had a sluggish starter.  turned out the brass bushing in the housing had worn out of round which caused the armature to not spin perfectly round and slow on each revolution.  It was an easy thing to check and reasonably easy to fix.  Remove the starter, use a mirror and bright light and look in the cavity at the bushing - lots of flecks?  Look out of round?  might be your problem.  of course I investigated everything else first like bad grounds etc etc and this was the last thing that I tried....worked great afterwards.
 
brett