Reducing the speed of the armored horses anymore than they already are, and they wouldn't be able to couch. You want to reduce maneuverability? Why? Don't claim realism because that is stupid, real horses are much more maneuverable and faster than their ingame counter-parts.
Reducing the maneuverability would force the horses to always attack from the rear, whereas now its only most of the time, anyone with enough athletics can dodge out of the way or take the easy route underneath the horse to escape.
If anything needs to be fixed, its the fact that the easiest way to dodge a horse is to throw yourself under it's hooves, for no damage if you are wearing anything more expensive than a mail shirt for minimal damage.
Of course people taking 30 agility for 10 riding and using champion coursers are going to zoom around the map, but they are utterly useless off a horse. It is no different than a 2h taking 30 strength, black armor, a great maul, max ps and if, then going to town as everything glances off, infact horses are the only way to counter those armored juggernauts.
The best fix for this is a class balancer, which could give even amounts of cavalry/ranged/melee to each team, to prevent all the cav and ranged being on one team, and all melee on the other.