It’s faster until you need the human operator to keep coming over because the anti-theft sensors keep getting tripped up by false positive readings. Or you need to find some vegetable code that a normal cashier has memorized.
Self checkout is great when it’s done well, and total shit when poorly executed. And unfortunately, it’s not always just a matter of technology (which normally keeps improving); it’s often a matter of business model: sometimes customer convenience is really important, other times loss prevention (which creates frustration) is more important.
I’ve seen countless good self-checkout experiences backslide into crap experience because the business felt that a controlled client is more profitable than a convenienced client.