Ideal speed for sonar to work?

[jbl_91762]

So after watching some vids on fish finders, Im gonna pull the trigger on a Lowrance Elite 7ti to replace my old Garmin Striker for my outback. I hope the 7" screen wont be too big and get in the way.... Anyhow I was watching a vid and a user said ideal speed in a boat is 3-4 mph for the DI and SI features to work the best. Well thats a pretty good clip for a kayak which the Outback has no problem doing but thats moving good. So what are your insights to this ideal speed as I cant find a vid on a yaker using it. Thanks