Your pump has a pressure switch at the base, when there is air in the system, the float will close the circuit, allowing the pump to turn on and fill the system. Once it is full, the float opens the circuit, the pump shuts off until demand allows it to turn on again. That's the schematic. The switch is either on or off, so your reduced flow is not that.
If it is the original pump, corrosion in the feeds to the pump, as well as the windings in the pump itself, could be resulting in a voltage drop that will allow the pump to operate, but at a reduced output. Most pumps will operate at 9-14VDC, but the lower voltage you go, the less the output will be.
And, of course, a mollusk may have found it's way into one of your lines...