Self checkout customers cannot verify their own age for age-restricted items.

Age verification happens asynchronously and causes zero delay for anyone who doesn’t look like a teenager. The employee overseeing the self-checkout gets an alert on their tablet-thingie, they take one look at me and press approve. You can just keep scanning items while this happens. Usually the ‘your age may be checked’ alert disappears within seconds.

Self checkout customers cannot scan something and report the number of duplicates (e.g., scan a can and punch in that you're buying 8 of them).

They can where I live.

In most stores, self checkout customers are policed by the system to make sure that each item is placed onto a scale that weighs everything, and stops the process if weights don't match up.

I’ve never seen that, and I’m not aware of any supermarket chain in my country that does this.

The ergonomics and flow of self checkout doesn't allow for a conveyor belt style rapid scanning, because a self checkout station is a tighter space and tends to require bagging as you scan, instead of scanning and bagging separately and independently.

The conveyor belt slows things down. You take an item out of your basket, scan it and put it in your bag in one go instead of it being two separate actions. You’re only handling each item once instead of twice. Besides, if you’re planning to get a lot of items you scan while shopping, not at checkout. You get a portable scanner, put it slot on your cart and just scan each item as you put it in your cart.

As a result, self checkout tends to be slower for customers who have more than 20 items.

If you scan while you add items to your cart it takes less than 10 seconds to check out, regardless of how many items you have

That might be offset if there's a longer line for regular cashier, but if there's no line the employee cashier is much faster.

My local supermarket has a grand total of 1 regular cashier, versus 16 self checkouts. If you go during a busy time you have to stand in line. Since the regular cashier is basically only used by people who don’t want to or can’t use self-checkout for some reason (that is: usually elderly people) this line doesn’t move very fast.

When it’s a quiet time of day there often isn’t a regular cashier at all and you have to ask the person overseeing the self-checkout who then has to call someone to help you out as they cannot leave the self-checkout isle unattended so you end up waiting for a cashier to arrive.

Self checkout is always faster, by an order of magnitude.