For a given number of cashiers on staff, having at least some self-checkouts makes everything move faster. If a self checkout takes twice as long while you're actually at it but a single cashier can run six of them, that's still three times as many customers handled by that cashier. Those numbers are made up, of course, but the point is that unless you're hiring so many cashiers that there are never any queues, it's not necessarily slower to have self-checkouts, it just shifts time from waiting in the queue to scanning items