I made the mistake of going to Walmart last night for a few groceries. There were 4 registers open and each had about 6 full carts lined up. 20 minute wait for three items. They close their express registers at 8 PM and have no self-checkout. My theory is that they want you to stand in line and watch Walmart commercials that are displayed in the line area.
I did the same thing a few weeks ago and had the same exact experience. They only have like 50 registers but 4 were open.