I agree.  There should be a maximum attainable speed of non-magic assisted units.  This maximum would also depend on the type of unit.  Horse units obviously are faster than infantry.

I agree it feels weird atm, but I think we would have to wait and see since we are missing 3 other tech trees atm. I dont think anyone can afford to go all-out warfare tech without being in trouble of someone getting some sick spell that would counter that fast moving army.

I can't justify the movement upgrades either.

Movement rate upgrades should be axed from research and Sovereign level ups. Human units should have a fixed movement rate determined by physical reality. Of course you'd still be able to speed up your units with mounts and magic. But that's where strategy and tactics come in instead of just "researching" your ground troops to march 3x faster which is silly. An army should be just as fast as its slowest unit and force tough tactical decisions on you.

That said, I would like to see a ranger/scout unit with a Pathfinding ability to lower movement penalties through difficult terrain.