I suggested testing the (or any) motor unloaded to take any other factors (mechanical, wiring, etc.) out of the equation.
I saw this person replace a starter,battery,alternator and positive cable to repair a slow cranking problem. I fixed it by installing shakeproof washers on the engine block ground.
Most auto parts stores perform a no-load test on starters when selling or providing warranty on them.
You can perform a load test on a starting system if you want.