I would also say the Costco self-serve food kiosk order system are fast as well
Costco is the only place where I've seen this. I don't understand how it gets an authorization for any amount that fast, since it can't know the total while the cashier is still scanning items, and it's Costco, so it could be anywhere from $50 to $5,000 so surely it's getting the authorization after the transaction finishes? The flow is almost perfect. I have them scan my Costco membership, I use tap to pay on the card reader, then I or 2nd cashier move to organizing the items into the cart, and then the cashier hands me a receipt with basically zero wasted time.
They actually ask the credit card processor to approve you for $avg + X%, so as long as your purchase comes in lower than that, you've already been approved. If you make a really big purchase it will take a little longer, because they go back for a second auth for the bigger amount.
It's also why you'll see some people making $700+ purchases without having to sign anything -- because Costco already knows they do that every week and pay the bill on time so they assume part of the risk.
I'm sure Costco got a deal that gives them nearly at cost processing and a bunch of other stuff.
It was worth it for Citi too. The moment I got my Costco Amex replaced with a Costco Citi, the Citi became my primary card, because everyone takes Visa.