Is the DB table chatroom_chat_online_list ever cleared? If not, it probably should be; it took up 17Mb of my DB (10Mb of which was indexes) with 143,026 rows, and had last-seen timestamps from over a year ago, before I truncated it. (I have the chat displaying as a newsbox on the front page of my site so it picks up a bunch of anonymous users, even though they can't type into it.)