I can't confirm this, but it seems pretty consistent
I'm in a decent fast sim, group chat starts up, suddenly I'm taking 30+seconds just to turn 5 dgrees, chats sent on the group window seem to come in late, out of order, take forever to show up when sent, and frequently just don't send at all.....
in each instance once the group chat starts, lag goes up, and packet loss goes through the roof (I've hit ~60% before with SL set at a 2Mbit pipe).... it seems to occur most with large groups
and as soon as the group IM is closed..... poof, all back to normal
anyone else seeing a similar pattern?