Based on what I’ve heard, it may be wasting time somewhere like Germany where the staff are scanning your items at lightning speed. However, here in Poland self-checkout is where you go to save time. Human-staffed checkouts are and have always been slow like an iceberg. Now and before self-checkout was ever a thing. I was relieved when they were introduced, precisely thanks to the time saved. I hate waiting in queues.
All self-checkout machines are moody and lock up if you so much as look at it funny. It would be somewhat acceptable if there was always a dedicated employee delegated solely to assist and unlock the machines, but stores cheap out even on that.
Now, you just need to deal with individual people being slow partially because it’s not their job and partially because it’s a lot less efficient to have a little kiosk vs a conveyer belt. Not to mention if you want to buy something that a store locks up you need to wait for them to go get it. Progress?
My pet peeve is automated checkouts and ordering kiosks. These save stores and venues on labor costs by making the customers do the work for the store. For free, and disproportionally wasting customers' time.
(In many cases this applies to self-service in general.)