Mine doesn't vary more than 1-2 MPH and when I speed up due to my palm resting too heavy of the cramp buster, the cruise slows to 1-MPH lower than set and then increases to the set speed.
Now it must be stated that I'm riding in flatland USA, but there are some inclines on roads I ridden and have not noticed my speed vary much at all (99% 1-up riding, but getting to the weight of 2-up

).
The throttle lock on my R80RT can't be trusted at all and is only used for wrist relief!
HTH,
Jer