If the power feels off and its not brgs/drag, about the only other thing it can be is it's been run hot enough to sap some of the magnets strength. I've seen brushless run hot enough to completely demag. Short of the rotor starting to come apart, there's really nothing in there. If its sealed, it shouldn'r be full of dust or dirt.

I did have a Feigao XL motor start to act like it was demagging, and I did puill that apart - rotor had unglued from the shaft and screwed up the magnetis inside. I did manage to fix that. Could always pop an endbell off and take a look.