I have bought single things using self-checkout that quickly, but I think the difference would be when you buy 5 or more things. Then the time to scan becomes noticeable.

Another, more subtle feature is that I never wait in line because I have trained myself to never go at peak times. I waste a certain amount of time thinking about it.

That said, I find the idea of a cashierless store super-creepy and will be among the last adopters.