One implicit benefit of the self-service checkouts is that there's often one line for multiple machines. Any student of queuing theory will know how much this reduces waiting time compared to the one queue per cashier that's otherwise common.
At least at Safeway I can spot the kind of person who looks like they value their time and it'll usually go faster.
I'll literally never shop at Ross again because it takes forever to exit their damned queue.
The model works better at Trader Joe's in SF where it's mostly people who value their time. You can then be confident all the counters won't be used by slow people.
I would honestly be fine with making that permanent. It's so much easier to pick the things I want with fewer people between me and the shelves. All in all a much more efficient shopping experience.
Is this really correct? Shouldn't one grand queue just decrease variance, not average?
Put another way: if the median is 1 minute, then half the time you'll wait less than 1 minute. The other half you might wait not just for 2 minutes, but significantly more. They won't "cancel out" to preserve the mean.