The switch just does on and off so if it's getting slower and slower it's most likely the motor or the power supply. You can try taking a multimeter to where the motor is getting power and find out how much power it is getting. Then check with a manual or some online source on how much voltage it should be getting.
Even though it's 80 bucks, you can return it if that's not the problem. They ususally give you a time frame where you can return items.