It boils down to how much you care about how things are sorted in your bags.

If you don't give a shit then all you have to spend time on in the bagging side of things is being careful placing in the bag products which are in glass containers (to avoid breaking them) and the rest you just drop in the bag. If you've essentially trained yourself to know the side of each product were the barcode is and the scanner is a front-and-bottom double scanner (same as cashiers) you'll be doing near cashier speeds at scanning, In that case and as you pointed out, there being more self-service tills means that if everybody was like you (i.e. don't care much about how their bags are organized, and like to optimize their own checkout speeds) then self-service tills would be faster.

Personally because I do a big weekly shopping and walk or cycle home from it, I actually have to have some organization in my shopping bags, and my bagging speed just about keeps up with the scanning speed of the cashier as long as I've ordered stuff properly in the conveyor belt that goes to the cashier.

As for multitasking, your example is like apples to oranges relative to mine - chewing is literally instinctive (requires no thinking and in fact if you try and think the steps of your chewing, it will be way slower), which is not at all comparable with my example were you have to look at things in different places (which you can't do at the same time because they're different places and you only have one set of eyes) and have to actually make a decision on the bagging side (i.e. "were shall I put this").

Also you seem to not know what an Assembly Line is - this stuff is totally independent of automation (it predates it, dating back to Henry Ford's time) and it's purely about dividing a complex process into multiple steps and have each person just do one step again and again. Since each individual only does one thing they become very effective at it, plus as they're working in parallel, the whole thing is much faster.

"Scanning & bagging" is naturally dividable because each it's own "attention + decision + movement" block.

But yeah, I agree that those systems you mentioned were people just scan things with a portable scanner as they pick them up from the shelves are much faster than manual cashiers because the scanning happens during shopping (there are also other systems were stuff is scanned all at once without having to taking out from the bag, thanks to RFID tags on the products, which are also much faster).

The problem is that the vast majority of self-checkout systems I've seen out there in two different countries are crappy little checkout tills with only room for a single bag, having a non-standardized UI (so, different in different stores) and a shit scanner (either a handheld one or a single-side scanner).

Rather than redoing the whole process to use the possibilities associated with the customer registering themselves the products they buy (as your example "scan the items when you pick them up from the shelves" does), the vast majority of self-checkout "solutions" out there just make the customer do the checkout in the same way as the cashier did, only with inferior tools than the cashier had (less space, less comfort, worse scanners) - so all they do is replace a professional with a free (for the store) amateur rather than make the whole process more convenient.

These "checkout just like a cashier but in a crappy station" self-checkout systems are just about ok for somebody familiar with computers who just wants to check out a handful of products, but are problematic when the store has, for example old people doing self-checkout of their weekly shopping.

In the real world not everybody is like you and when there are no cashier checkouts available, these self-checkouts which are really just inferior versions of cashier tills tend to end up with old people, computer illiterate people, people who need to pack their bags in a certain way and people with lots of shopping gumming up the system because they're way slower than cashiers, at which point you lost the speed gains from having more self-checkouts than you had cashiers and the self-checkout tills end up with queues just like the cashier tills used to have. If you have mixed systems, then people who would be much slower if doing self-checkout will go to the cashiers and the rest can go to the self-checkouts, but the ratio of one to the other needs to be properly balanced (and I've seen several cases of it not properly balanced)