Lanza expended 154 rounds in five minutes. He reloaded six times.
If he had only ten round magazines he would have had to carry 15 additional magazines
instead of the nine he carried in order to expend the same number of rounds.
Allowing four seconds per reload (and that's generous) Lanza would have needed an additional 40 seconds at most
to do what he did, probably less, since half of the magazines weren't empty.
Banning high capacity magazines won't solve a thing.