Self service checkout does not speed up the time it takes you to check out, but it can (and usually does IME) dramatically reduce the amount of time you wait in line to begin checking out.
When a store has a single full-service line staffed and four self-service lines open, the presence of those self-service lines certainly seems to improve the situation, vs a single full-service line. But what if it were instead compared to five full-service lines? The five full-service lines would doubtlessly be the fastest.
Sure, but the supermarket would have to pay to staff those other four lines, and they've decided they'd rather pay to maintain self-service machines instead.
I'd consider using those machines if they gave me a discount. But as it is, they're slower, exploit my labor, and are used to suppress the wages stores pay out to members of the community (making regional wealth extraction more efficient.) And worse than any of that, the machines are pedantic and finicky.
I see no conceivable upside. I totally get why companies are installing them; it's plain old greed. There is nothing complicated about that. But knowing that doesn't make me want to use them.