I suppose to me the biggest question is that which PuppyDogGeneral already mentioned: who is in control of Yata Mirror? Is the Mirror reacting on its own to oncoming attacks, or is Itachi himself altering it's size to defend himself? I believe it is the latter. Why? Because if it were the Yata Mirror reacting to oncoming attacks, my belief is that it would expand to the perfect size to block the attack, and nothing more. However, in the scan you provided where Sasuke throws the paper bombs attached to kunai, the shield expands more than necessary. This is, of course, mere speculation, but it seems to me as though it reacts according to the user, and so if the user does not see an attack (this relates to the speed argument), then the shield will also not react. Similarly, the user would have to see an attack coming to block it. This seems to make more sense in my opinion.