For one of the supermarket chains I frequent, from me handing over the single item I'm purchasing I can be walking away within 20s. They scan the item, then a loyalty barcode on my phone and finally I tap my iPhone which has my bank card loaded onto it.

Basically, they don't have any significant bottlenecks, POS hardware + software is responsive with an efficient UI for the operator, and the card machine is a wired connection to a router with a fibre uplink.