As always it depends on what you want to measure. Comparing total checkout time including queuing though for 'speed (items/s)' is going to naturally favor the self-checkout because the choice between normal and self checkout is heavily weighted by the number of items a particular person has. Anecdotally I've never seen anyone go through self checkout with a full cart but see it all the time with the normal registers. Additionally there's rarely the same number of traditional registers actually staffed compared to the number of self checkouts.