Brakes, rotor, braking, lose, you're, and so on. I feel better now...
Brushless motors wear the same way as the brushed motors used to, with the obvious exception of the brushes and the commutator. That means the magnets in the rotor, and the bearings in the can.
The magnets will lose strength over time due to the heat they have to cope with. That's made worse if you use the brakes a lot. As their strength reduces, the brakes will be less effective. In a BL motor the brakes are applied by firing the coils against the rotor rotation, and that generates a lot of heat. The same applies if you use a lot of timing-on-the-fly (boost and turbo).
Bearings usually last a long time. They're easy to check, just pop the rotor in the bearing when it's outside the coils and spin it. If it spins the bearing is fine, if not it can be replaced.
The most likely explanation is that the rotor has lost some of its magnetic strength and thus the braking performance is reduced. If the motor is a popular type, the rotor has value to people running blinky classes, especially in 1S, as a low-strength rotor is actually a plus. I'd try checking the speedo setting first as metal mickey suggests, and also confirming that the speedo is still applying brakes. Put the motor in a mates car and see if you get the same result.
If it is the same, then a new rotor is much less than the cost of a new motor, and would be the fix of choice. HTH
