You aren't faster at checking it out per item. The supermarkets are just keeping it understaffed all the time so the human checkout has a long wait while self checkout does not, so it's faster to not wait. I try to not use self checkout, but if I have a few items it just makes me feel guilty about making other people wait behind me.

As they keep having understaffed situations, sometimes there's no other option except self checkout. Which honestly should be illegal.