Hot take: self-checkout with 12 items beats the express lane with 3, and I'll die on this hill
So my local Kroger on 5th has both self-checkout and a 10 items or less lane, and I've been running a little experiment for about 6 weeks now. Every trip I time myself from the moment I pick a line to the moment I'm bagged and walking out. Self-checkout with a full basket of 12 items has averaged around 4 minutes. The express lane with just 3 items has averaged closer to 7, mostly because the person ahead of me always has a coupon issue or wants to chat with the cashier about their cat. The one time I beat it was a Tuesday morning when nobody was in the store at all. Part of me thinks self-checkout is antisocial and I should talk to humans more. The other part of me has 47 minutes of my life back. Which side are you on here, and does it change if the cashier is actually fast?
Wait, is 4 minutes really the full time though? In my experience the self-checkout timer is a sneaky thing. You're watching your own clock but you forget the time you spent waiting for a machine to open up, or flagging down the one worker to fix the "unexpected item in bagging area" alert. Your mileage may vary, but I've had a 12 item run take 4 minutes of scanning and then another 5 standing there with my hand up like I'm hailing a cab. The express lane number is probably honest because the clock starts when you join the line, not when you start scanning. So I'd say measure both the same way before you die on that hill, because I bet the gap gets a lot smaller.