[dsraven]

when it cranks over does it sound like it is turning as fast as it should be? check all the cables full length for bulges etc that would indicate corrosion inside the cable sheath. they may not be as bendy in some spots either which can indicate corrosion under the sheathing and poor conductivity. you could also do a voltage drop test to see if the cables are conducting as they should be with no resistance from corrosion or poor connections. if all connections, cables etc are good and it still cranks slow then I would remove it and have it checked out for rotational speed free running etc. a starter rebuild shop will know the specs and be able to test it.
if it is a key start, not a pedal on the floor, then it could be a bad solenoid, inside the solenoid is an electromagnet that is energized by the key and pulls in a plunger that contacts 2 points together. sort of like a big washer on the end of a plunger that is drawn back against two contact lugs. one lug is battery power and the other lug goes to the motor. that is how the starter gets power to the motor. anyway, over time or non-use the contacts can get corroded or worn so there is not a good contact anymore. the result is a starter with less power to turn because it is not seeing full voltage due to the resistance of the connection in the solenoid. the solenoid also pushes the starter gear out into the flywheel teeth, if it is a key style starter system, so if it isn't getting full power it may not contact all the way into the gear on the flywheel.
a pedal on the floor style starter has a mechanism that pushes the gear into the flywheel ring gear as it also pushes a switch, mechanically, to energize the starter motor. the contacts can get dirty or corroded as well as the linkage can also be out of adjustment so the gear is not fully engaging the flywheel ring gear. one issue could be a bad engine mount allowing the engine to move around which can affect the linkage. check it all out then get back to us. some pics would help.