Surely the same happens with cashless payments? A teller in a store can process way more transactions via contactless than with cash in the same amount of time.

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.

That question of "does this allow for more work per unit of time" is honestly a pretty interesting way to stratefy whether an innovation is actually innovative imo. For example, the first iPhone? Sure, you can email and video chat on the go and conceivably do more work over time. But the 14th compared to the 13th? No, all that effort spent didn't really unlock more work per unit of time. If anything you are doing the same work, at the cost of more resources because the hardware is more overpowered, the screen has more pixels, and the bulk of the resource load on the hardware is from rendering stuff like sexy window dressing instead of the actual functional process, like checking your email.

Pretty interesting parallels to biology too, like in some birds there's the race towards sexually selecting ever fancier tail feathers that might cause more female birds to ooh and aah perhaps, but hurt your chances of flying away from a hawk, and lead to an overall reduction in fitness in the species as the hawks start finding more success and expanding their population size at the expense of yours.