Based on my Humminbird I stroll along up to 4mph just fine. If I sprint I'm just shy of 6mph and only bigger or fresher macks do fine. Weaker ones spin and twist but it was probably time to change those out anyway.
I do stroll along at 2.5-3mph on average when I'm focused on dragging a mack in the zone. Usually looking for the usual signs. Most of my hits are around this speed.
P.S. that could be in knots. All of a sudden I forget what settings I have.