Review on Freud Dia Mortising Shank 16 106 by Tom Tofte

Bit appears to have lost its sharpness after about 15 linear feet into MDF. First off I'm new to routing so this could be caused by me. I had to run a duct about 15 feet from the MDF shelves. The punch got so hot that if I stopped for a moment the wood would burn out in about 20 inches. Speed was kept below the maximum rated 24,000 rpm. The channel was only 3/8 inch deep. I'm not sure if I'm doing something wrong or if I need to find a better piece. Open to suggestions.