What's the other speed(s) it runs at? What sort of carpet have you tried it on? Domestic type or low profile?
When it bogs down, does it cut out and you have to re-set the trip switch? Or just stop turning and you have to lift it off to get it going again?
Ideal speed is in the 180-230 range so 300 is a bit fast.
I have a buffing machine that's about 350-400rpm (goes round too quick to count!) and I tried it on carpets. It cuts out... faster machines are for buffing floors, their motors simply don't have the torque to push against the friction you get on carpets.