Um, MOMX/MOMY in devmode show your real speed without taking conveyors into account. In previous 2.1.x versions, the code for normalizing speed was bugged and did take conveyor momentum into account when it shouldn't have, limiting your maximum speed to your "normal" speed plus whatever speed the conveyor was going when running with the conveyor belt.
Conveyors very much do affect your movement at any speed; it's not hard to test, just time yourself running from one end of the conveyor to the other, and then repeat it going the other direction.