I didn't have any issues with lag whilst playing multiplayer at home (my connection is not great either); but I must admit that whilst on holiday when the Vita would connect to PSN on the hotel public network, the game would not even connect to the KZM servers and would quickly time out.
I'm really enjoying the additional contracts objectives; they made me explore each level a little more than my initial play through, which was a bit gung-ho if I'm honest ;-)