I just spent a good 5 minutes or so actually making this example in my test map and playing it. I guess I was curious to double check the answer myself, even though it ended up proving what I originally had thought.
I made 2 science vessals, one hero and one non-hero. A burrowed lurker was constantly being moved under the non hero science vessal. In this instance, both vessals traveled at exactly the same speed.
Based on this, I would also guess that air units being centered on ground units would also not slow them down, but I am not going to bother to test that.
I hope this was what you were looking for

EDIT: saw you post after I did, I had my window open from when I originally saw it.
I do not believe there is a way to slow down an air unit, besides choose a slower air unit and force the valk to constantly be moved to that unit
