#2- Overdrive seems reasonable to me.
That got me thinking that the belt drive might help compensate for any resistance that might occur to the brush roll from thick carpeting, or to the air flow from the bag becoming filled. If the fan is turning faster than it needs to be, then perhaps it maintains a minimum level of efficiency throughout the filling of the entire bag? If there’s one thing that my X series upright does very well, it still cleans well even when the bag is packed.