This entirely depends on the quality and calibration of the self checkout machine. My local grocery store has very well maintained machines, and I can personally scan items a tad bit faster than the experienced checkers who do it all day long. However, when I go to other stores, especially non-grocery stores, the machines are slow as hell and have that "tutorial" mentality that you describe. The one at my local grocery store has the ability to bypass every single thing it says. It could be mid-talking, and you can already scan 2-3 items ahead of what its reading. It'll ask questions like "choose a payment method", but if your credit card is already in the machine, the voice line plays, but the prompt clears itself out automatically by detecting the card reader's processing.
As far as your thought on polling: my particular grocery store, even if NOBODY is in line for the checkers, the self checkout will still have people queue up. It is by far and away the preferred method here. But admittedly, I also live in an area that is mostly tech workers, so we're not afraid of using a screen and buttons ourselves, since that's our day jobs.