In my experience, the slow down is the cost of the extra features of the self-checkout system running on (I'm guessing) weak hardware. There is a rather large delay when choosing payment sources or in between bagging items.
Between bagging items, it has to wait for the scales underneath the bagging area to register a consistent weight. When choosing a payment source, it probably has to do a network call to the PIN-pad system. It's not like it's running on a slow CPU.
