Too much current draw, loss of grunt and or speed with a starter when it gets hot usually indicates that the insulation between the windings on either field windings or armature windings has deteriorated.

Was the starter overheated recently?

sounds like the solenoid contacts could do with a clean up with file or emery cloth then

And maybe check the solenoid winding, could be he's failing when hot.

This was a common shovelhead issue overcome by running a heavy guage wire from the battery through a 35 amp starter button and straight to the solenoid,you will need to disconnect the other wire from the starter button to the solenoid and wrap it in tape, let us know how you go.

the fatter wire is the go + keep the relay close to the solenoid also  , if its & aftermarket solenoid there not the best , @ 10.5 cr  what cam is in her ?