I've mixed on all speeds. I'd put the 20-qt on low if I needed to walk next door to the Family Dollar to get something, or walk across the lot to Burger King for lunch. I'll put it on medium if I'm doing things in the kitchen.
Or i"d put it on "Beat the hell out of it!" if I was in a hurry!
It's one reason I like this recipe. There are no rules for mixing it.