I think fast or slow works, as long as the speed of the needle coordinates/matches the speed with which you move your fabric! I often wish my machine had a speed regulator on the presser foot-- cruise control! I can find a comfortable speed, but holding it steady can be a challenge.
They do say "practice practice practice" but I'm not patient enough for a lot of that, but right now am trying Ricky's machine trapunto with water-soluble thread, and am using that for my practice! Even though this is the "real quilt" my stitches aren't going to totally matter b/c they'll disappear, and by the time I'm done with all of the trapunto, hopefully I'll be much steadier when I get to the "real thread" work! It's so forgiving! Florence