I’m slower also, but I still think it’s worth doing myself to avoid interaction.
They'd really rather you not get in their way, anyway. It isn't awkward for them until you try to help. They enjoy the feeling of power/superiority they have over you. That's why they smile while you stand there naval gazing.
A checkout clerk just feels like a weird adjunct to the whole process. I walked to the store, pushed the cart around and grabbed all my items, yet somehow someone else has to do the quickest and easiest part for me?