Since e.1.5.5 shieldwalls make your soldiers "extremely" slow. It makes shieldwalls allmost unusable, because they get shot long before they reach anyone. The more arrows the enemy can shoot, the more chance he has to hit your troops, despite the shields. As a comparison you can see myself running with the shield infront. The player character is not effected by that, running slower as usual but still way faster than the other troops.