My wife works in Waitrose, multitasking Checkout, Cafe, Replen, they are very tight staffed she is doing 2 extra shifts this week. There also seems to be supply issues, some items are in short supply especially Mondays.
Customers have the choice of scanned, self checkout or tills, the self checkouts are very popular and always busy, there is also an express till for a few items.
When I’ve been there if there is a long queue an extra till is opened, my own grumble is the discount offers, to get them you have scroll through your phone app to redeem them, a slower system would be impossible to devise, that slows down checkout a lot