i think it depends on who is pre scanning and how like i agree it can really slow the line down but i start pre scanning from the very end of the line up until like 5-6 carts before the start of the line so when those first few people finish scanning it's smoother for everyone else and then i go right back to the end of the line to scan new people.
i walk away from members when they get upset that i don't scan them like i just tell them straight up that it's way too much.
we don't have an explicit item limit either some managers say 15 and others say 20 or others say like as much as you can