"How would it be more efficient than a traditional checkout? There's no way the average person can match the speed of a cashier who does that every day, and at least the supermarkets I've visited don't have enough self checkout machines to balance that out."

Where I live, the speed of a cashier is not the bottleneck. The bottleneck is waiting in the line after 3-4 people, especially if one of them has some issue "eh, I didn't want Foo, I wanted bar, let me run for it real quick".

Self-checkout parallizes the process, and if a single "Exception" is thrown, the other threads still run just fine.