Dividing speech into good/bad isn’t going to help when you have to understand a name, address, price, involved situation, or anything else important. If I don’t understand necessary information someone is telling me, I’ll say, “No le entendi. ´iRepita, por favor,” or “Otra vez, por favor,” or, as a final attempt at understanding, “Escríbalo, por favor.”
Meet my friend Rolf.
One day in class he passed around his drawing of a most wonderful invention: the Despaciador. It was a remote control to be aimed at anyone speaking rapid Spanish. The buttons to press were “Slow,” “Pause,” and “Repeat.” Of course, you can say, “ Más despacio, por favor,” but wouldn’t it be more fun to have a Despaciador?