forums_message__root_message_id is defined iscachable on newer versions of Forums. The version on OpenACS.org was not. I replaced the function with the newer one.
Using the function to calculate the root message is approximately 30% slower than doing a join on the forums messages table and using tree_sortkey to calculate the root message. Its about 1000% faster using the newer version of the function.
Thanks for taking the time and responsibility to fix this problem. I don't think I'm exagerating when I say that you just helped *every* user of this site and probably will cause uncounted new members to *stay* once they find us.