Possibly a reduction in movement rate is fine but if it impacts your OB it means that nobody wears heavy armour.
My players never complained so much about the OB penalty, probably because by the time they were able to afford buying a set of plate armor they were fully skilled in it.
If you're skilled enough, Plate Armor actually enhance your OB, if you compare it with leather armor:
Leather Armor (soft) gives you a +20 to DB and +0 to OB
Plate Armor gives you a +60 to DB and -20 to OB
That means that a fighter wearing leather armor must spend 40 points of his OB to have the same DB than one wearing Plate Armor. So a fighter wearing Plate will have 20 points of OB more than one wearing leather usable to attack (40 -20 of penalty), because armor protect him.