At first I found the packing your own bags to be a real hassle (my US home is a small town and the cashiers are taught how to pack bags correctly - when there isn't a "bagboy" available, which there usually is -, they always load them into your cart and usually engages in pleasant conversation during) but like you, I found that the cashiers just toss everything into the bag with no consideration at all. Then I realized that packing my bags was much more practical regardless of how long it took. Oh, I do put my items on the cart in the 'correct' order (i.e. frozen stuff together, boxes together, cans together, bleach or cleaning products together, etc) and it still gets jumbled up. I remember getting home one time to find that my bread was in the bottom of a bag (and I always put my bread on the conveyor belt dead last so I have yet to figure out how that happened), a 2L bottle of pink lemonade on top of that and bottle of toilet cleaner beside that with package of mini muffins on top. Who the hell does that?
Aside from the that, I always felt rushed in grocery stores in the UK. The cashiers just seem to have the worst attitude ever. I always felt like the were running late for a really important date with some really hot guy waiting outside for them and somehow doing the job they are getting paid to do is a slow death.
I've had service like that in the US also but I guess I've been spoilt by the little old men who work in my local grocery store (in the US).