I would pull the bottom plate off the pump and see if anything has lodged in there. It has been a long time since I had one apart but I believe there is a built in relief in those. Check filter if you have one, pull and inspect the regulator last.
If you are running the pump off of a long wire from a switch like a lot of people do, make sure none of the connections have corroded and causing a voltage drop. I have always put a relay by the pump and run a larger wire to the relay from the battery so the switch only carries the relay coil. It will can a huge difference in the speed of the pump.
Just a few things off the top of my head.
Curtis