When you have a full cart or two, the speedier scanning at a traditional checkout pays off. But usually both are available, not just self checkout, so you're free to choose when that isn't the case.
The big Tesco nearby has a single combined queue for all the 20 or so self-scan checkouts. They are rarely all in service. With a few items it is now always faster to go to an operator checkout unless you go in at 4am. The express lanes of "under 10 items" or "cash only" are a memory of a former, much faster time.
There is often an employee aggressively marshalling people to the self-scan checkouts at every available opportunity and trying to pull them away from the staffed lines.
In the local Pound shop they tried replacing all checkouts with self-scan, getting rid of all others. They had to reverse this a month or two later and put back half a dozen, now brand new, staff operated tills - ripping out a few weeks-old self-scanners to make space. They are again slower thanks to rarely having staff nearby to handle the steady stream of staff authorisations and errors, so there's a big delay every time someone wanders over from the back of the store to wave their bar code and immediately disappear again.
Self-checkout is all about cutting labor hours, period. It is both slower and leads to much higher shrink (as much as 150% more) than a manned casher. Stores hope to make more money by keeping the store open longer. The hacks around the slowness of checkout without cashiers is for people optimizing for time is online ordering or shopping at midnight.
That is exactly the case. If you pay 10$ an hour for 3 shifts 8 hours each, this makes 3x8x10 = $240 a day. That is $7200 in 30 days per one checkout spot. To setup equipment for self-checkout and maintain it order of magnitude cheaper compared to self checkout. Another factor is eliminating managerial overhead while dealing with human beings, as one store manager told me referring to self-checkouts: "They are never sick, they do not have an attitude, they do not quit and they can't be rude to customers".
Self-checkouts trade one kind of unhappiness for another, and many (most?) are horrifying experiences. There are a few that are reasonably quick and user-friendly, but certainly not all. Most seem to be scraping the bottom of the barrel in an effort to squeeze a few more $ off the purchase price.
If you scanned less than 20/minute you had about a month to get to 30 before you were either canned or lost hours.
Things might be faster now with the unmanned ones, but the contention is that that’s only because they opened more checkout terminals. Granted, they wouldnt have done that with staffed ones because of the increased expense, but let’s not get confused about the real cause.
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Maybe if you're selling a checkout system. Personally I care how fast I can go from "got things" to "left building". (or maybe more accurately: left home -> returned home, but that has a lot more variables)