It depends but generally SLOW. Sometimes you are speaking FAST and not stuttering at all and that is good but it's only happening because you found the right rythem, if something stops that rythem you'll probably stutter. If you always speak SLOW and with a certain rythem you'll improve, trust me. It may sound boring but at least you are not stuttering. One way that helps me is, while i'm speaking i set the pace with my finger. Try it on a table to hear the noise and speak along.