Complicated even more by shopper loyalty programs.

Following Amazon's acquisition of Whole Foods, now checking out involves an extra step: the cashier asks every patron "Are you a Prime member?"

Then a number of things can happen. Worst case scenario: they are a Prime member but haven't installed the Whole Foods app on their phone. Then they are encouraged to find and download the app, install it, open it, find their login info and enter it, then bring up their unique barcode on their phone for the cashier to scan. In high-volume stores this adds ages to wait time.

Another retailer near me (Uniqlo) also has an app-based loyalty program and I've experienced the same delays there.