Self checkout would be a great idea, if you used the same software the cashiers used.
Unfortunately, like every software made since 2010, its gimped so that it doesn't scare people, so its too slow. I used a scanner at home to scan my books (they are just usb keyboards, so surprisingly easy to integrate in e.g a spreadsheet), and the scan was so quick. I could go through all my grocecies in basically no time if all I had to was scan them.
So I always pick the cashier, and then start reading on my phone while standing in line. This essentially reduces the wasted time close to zero, even if it does not reduce the wall time to zero.