The only thing I can think of is that for some reason building the inital IAL from the server responses upon entering the room is somehow very intensive, and thus overall bad?
nope, the irc server doesnt give the infomation needed for the IAL when u join the room, thats why its not populated.
Of the top of my head the only thing the irc server tells u are the names of the nicks in the room.
thats why a /who $chan on join is needed, to get the rest of needed info.