Resident in Sweden here and heavy user of the self-checkout. My experience is that self-checkout is slightly slower than the serviced one. For a serviced checkout I try to put all bar-codes towards the cashiers scanner (it is a nice thing to do, the cashiers do not like juggling either) when putting stuff on the belt and I will need to unpack from my basket, pay and pack it to my bag. The same is operations needs to be done for the self-checkout but even the best setups needs some managing of the machine that the cashier would have parallelized in a serviced one. Usually it is slightly more fiddly to do packaging and scanning on a self-checkout system than on a serviced larger belt.

However, considering that there are usually many more self-checkout terminals available with much less queue it means that the total time spent is less than on a service one. My local store, open until 23, usually closes down the self-checkout terminals the last hour since there are too few people to make it worthwhile and you rarely have a queue to the serviced station.

So I totally agree that the serf service checkout is slower for that actual station but considering that the store can cram in much more of those in the same space and they need much less personnel to manage it saves everyone time and money to use them, unless there is no queue to the serviced checkout (and personnel is manning it at the moment).