That's the moment I would have said....
"could you hold on a moment please, I forgot something".
I'd say excuse me to the rude woman behind you--then go the long way to customer service and request the manager.
I would explain to the manager that I noticed this awful thing that he might want to check out--and escort him back to the register. I would then explain that I noticed that the customer behind me and his employee seemed to not like the manner in which I unload my grocery cart. I would then explain that if their store doesn't like the speed at which their customers unload, then they might wish to hire extra help. I understand that the cashiers get "scored" on their scan speed, but I'm not at all interested in improving her score while compromising the service I received and being spoken to in such a disrespectful manner.
All this of course to annoy all those around me.
Geesh!
I've worked in cashiering, my scan speed wasn't lightening speed and it always irritated me that they wanted faster faster faster--but I'll be darned if I'm going to talk down to a customer and suggest that they are too slow.
It ain't Supermarket Sweep for goodness sakes!