Think of a solenoid as a switch, on or off, open or closed. The bendex in the starter is probably going out. When the solenoid is energized, it supplies power to the starter and turns, the speed then spins the bendex gear out to make contact with the flex plate. It is possible for the solenoid to go out, but unlikely to cut the 12v power down so it would say turn slow, more likely you will need to replace the starter.