starter cranking slowly can be due to voltage drop at some connection in the circuit through which the high current flows to the motor ie battery terminals (must be clean inside and tight), earth strap connection to car, cable to solenoid and motor casing to bell housing. Re the last 3: a firm mechanical connection can hide a lousy electrical connection. Take off the bolted lug and clean it. Check that the motor bolts are tight and that paint isnt insulating the motor from the bell housing (this is your return path to battery negative). A neighbour of mine fittted a recond starter that didnt work at all simply because of the lovely black paint applied in the workshop which effectively insulated it electrically from the bell housing. Remember very high current is required by the starter so even small resistance is significant: v (voltage loss) = i (amps) x resistance (ohms)