This is only true because of having to wait in line for the checkout personnel. Once you get to the person, if they're even reasonably skilled, they can check out your groceries faster than you can.

My local grocery store has something like 15 checkout aisles, and usually only have one or two open. If they manned each aisle, there would be no wait and self checkout would be pointless. But they are not going to staff properly because the CEO needs another yacht.

In Germany, the entire self-checkout section moves as fast as one human cashier (they're very skilled, this is no joke). And they have one human supervising it at all times. And it takes the space of two human cashier lines, so they double up one of the other lanes. At the end nothing is really gained except a little bit of privacy (but not really because the supervisor's terminal still shows everything you scan)