Definitely depends on where you are. In cities or in California in general they are terrible compared to the rural Midwest (where they started and land/labor is much cheaper).

Every Walmart I've been to they're shuffling people out at pretty impressive speeds.

Next thing, you're going to tell me Santa is real ;). Every Walmart I've ever gone to, their checkout system is terribly slow. Their self checkout is better but still gets overwhelmed easily.

Do you have a Walmart Super Center? I wonder if it has to do with store size.

I do. There are actually a few different kinds of Walmarts around that I have visited, and it's generally the same story. I think it's a time based thing, not so much size, IMO. It seems like no matter how large or what they offer, their prices draw crowds, and they both understaff, and either under train, or make sure to put the slowest humans on Earth as the cashiers.

But the time criteria is best said in terms of when to go, since that's a much narrower window than when not to go.