The old client works fine, clockwork, but the new one is just horrible when it comes to speed and reaction time. Don't get me started on champion select, because that's where I get maybe 5 fps during the whole thing and it's very annoying, I can't even check and change masteries in time. It's a lot of pain.
I do hope the official client will not have this problem as then I'll have to reconsider playing LoL anymore.
Don't ruin my favorite game.