If it use to work and is now slow, take the prop off and check for fishing line. Minn Kota says we should really do this after each outing, right?
Check the voltage drop in you wiring system. (1 - Voltage at motor) ÷ voltage at battery = should be less than 0.05 or 5%.
95 Skeeter 150ZX w/ Merc XR6 150
Ocean Kayak Prowler 13
59 Yellow Jacket w/ Merc 50