Well, as I've wrote somewhere else, I have some minor expertise in evaluating business processes because of my job and I lived in London (UK) during the time when the largest supermarkt chains really started pushing self-checkout (the retail market is quite concentrated there)

I've also seen some attempts at that done were I am now, Portugal, in some supermarkets as well as how cashier operation can be done extremelly efficiently at the Lidl supermarket I used to shop in when living in Berlin.

From my observation when they install self-checkouts the supermarkets will not only reduce the numbers of manned tills but seem to purposefully reduce the number of people manning the available tills so as to push people to use self-checkout, and this happenned both in that transition in the UK and here in Portugal for the few supermarkets trying that, all noticeable because checkout via manned tills becomes slower when self-checkout is introduced compared to before that, something which logically does not make much sense without the "pushing people to use self-checkout" motivation (logically, with more tills in total waiting times at any till, manned or not, would become shorter, not longer).

Also there's an interesting psychological effect applicable here which is that time passing when waiting feels a lot longer than time passing when busy (such as when doing the actual self-checkout), which in this situation means that even if the total time from queuing to leaving is longer because it takes you longer to scan all articles and pay, it can feel shorter if the waiting time is shorter.

(A interesting "sciency" experiement here is to actually measure it with a chronometer)

So yeah, if you look at it from an "in the moment" and not at all systemic point of view, it does seem that when there are self-checkouts they're faster than manned tills, both because you're not really counting operating-time like you count waiting-time and because the supermarkets seem to very purposefully underman their tills when introducing self-checkout to push people into self-checkout, probably because their long-term objective is to cut down on manpower hence boost profits (which is also why they won't give you a discount for using self-checkout as that would go against the whole profit-enhancing motivation).