First of all, check the voltage at the battery posts, then at the output terminal of the starter solenoid (the big lower terminal of the solenoid) and see if there's a difference in those readings. Quite often, slow cranking is a result of too much voltage loss before the current gets to the starter- - - - -bad cables, dirty connections, etc. If the voltage drops below 10 volts while cranking, try a known good battery in the circuit before condemning the starter.
Jerry
