But statistically, there will still be a shorter average wait. If there's a slow customer on one machine, the line only slows slightly as the other five machines are still moving along as usual. And people tend to use the self-checkout when they have fewer items (generally), which means less wait per person.

Even if you have lots of slow customers, the wait will generally not be any longer. If the wait is longer than the staffed checkout, some people will move over and the line self-regulates. And even if it doesn't, there are still lots of slow/chatty checkout staff (at least where I live), so I don't think they're any faster.

Unless you go to Aldi. Those people mean business.