CA are making mediaval primarily as a game, and therefore want it to look exciting and fast paced, they arent gonna care about movement speeds, because to be honest if i was having a battle with knights, and they took 10 minutes to reach the enemy because they were more realistic, i would probably tire of the battles rather quickly.

Also, the battlefields may be much bigger, so they will have to increase unit speeds to avoid boredom.

Also, in the promotion video, they probably increased speed to allow for enough action.