Why do my Tama Iron Cobras make me too tired?

I don't know what it is about these pedals, but I get tired SO quickly from using them. My legs just die. I know it's not ALL me because my friend has cheap $100 Ludwig pedals and I can play those just fine. Faster too. My bass is tuned really tightly, I use the normal felt IC beaters, and the springs on the pedals are extremely loose. I also have duct tape wrapped tightly around the beaters to sorta condense them an make them tighter for more bounce. They've also hardly been used. Any suggestions?