That works until there's ~only self-checkouts, then it gets slow again.

There's a knack to using them, and unless you have a generous number of them (I know of only two local stores that do), you end up getting stuck while the single staff member assigned to the self-checkouts assists customers having issues.

Plus it's not unusual for a significant number of them to be out of order, not accepting credit cards, only accepting credit cards, or just being generally temperamental.