Can confirm, in most supermarket, paying take literally one second. You pull out your phone and show your Alipay/WeChat QR code, the cashier scans it and 1 second later it's done.
People paying in cash is a small minority
Speeding up lines, they also can have more clients for less cashiers
In France, most people pay with a bank card, the rest with cash. The process is longer and less user friendly