I agree that contact-less transactions are able to be performed faster. In the situation of a grocery-store checkout there are other factors at play. Have you paid and everything is still being bagged up? Is there anyone waiting on you? Probably others I'm not considering here.

Essentially I think this gets into a estimation of magnitude problem (outside of ethical, security, and other concerns). Where if the payment isn't ever the action being waited on, a contact-less payment while convenient doesn't save you any time to get out of the store. If you have just a snack and no lines then the payment will be the action causing the bottleneck.