I understand what you are saying, but a good machine won't need to spin it that fast to find the weight difference.  Just think of a tire balance machine, it turns the tire what may be equivalent to 35mph or so but will balance the tire to highway speeds and beyond.  Heck, for that matter, I could balance a tire with a bubble balance without spinning it at all, and probably could do the same for the fan if it were off the shaft.