The key is to just dump everything back into the cart and bag it later, on the table behind the cashiers. I sort my stuff by how well it can be stacked - pasta, cans, potatoes and the like go first, then milk, cheese, etc, and finally fruit, delicate vegetables, crisps, and other things you don't want crushed. You can just shove it all in the cart that way - I can match the cashier's speed.