There are a few more components than just the starter ie cables starter relay etc. Poor connections on battery cables or small or worn out cables will really slow a starter. So will an old near end of life battery.

Check battery voltage while cranking. Should only decrease to 75% of open circuit. Check for volt drop from battery to starter solenoid and from there to starter. Check volt drop from battery to ground. Do all these while cranking BUT let starter cool for a minute or two between tests.

Dan Bentler