My 920, still on Black, is still smooth as glass after the update, even after I created 17 live folders on my start page. Since I'm pretty sure I'm not the only one without this problem, I suspect that those of you who have it either have a problem with the way the update installed on your phones or you have some app incompatibility problem. If it's an install issue, maybe a hard reset will fix it, although maybe some of you have already tried that?
BTW, I don't have the Cortana tile pinned, but obviously that isn't the issue for everybody, either.