What causes a starter to crank slow?
In this regard, can a bad starter cause slow cranking?
A starter that's failing may crank the engine too slowly for a quick start, or it may not crank the engine at all. Often, the problem is not the starter but a low battery or a loose or corroded battery cable connection. If the battery's voltage is low and it doesn't take a charge, your customer needs a new battery.
Accordingly, what causes a starter to turn slow?
Slow cranking could be the result of several possible conditions, and as you've suggested, the fault might reappear or worsen. Possible causes are a degraded or defective battery, faulty electrical connection, a charging system problem, a bad starter or battery rundown when parked.
During starting of this motor under load, the high initial current, due to low resistance and no back emf, produces a very strong magnetic field and consequently high initial torque. This characteristic allows the series wound motor to be an ideal starter motor.