It's always demeaning. Calling a full-grown man of any race "boy" is belittling them. Yes, there's a special racist association, but it's been used as much on white men. The female equivalent might be "little girl."
"What do you think you're doing, little girl?"
It might have the same effect as simply "girl" if said the right way, but "girl" has been more normalized and sexualized, so it's a little different.
Anyway, the terms are belittling, and therefore demeaning, regardless of race. The point of using them is to position yourself over that person, as a parent over a child; it's shorthand for saying they are beneath you.