While I understand that the problem may not be the size of the forums (after all the largets forums are also the ones getting the most traffic on any given day), but is there a particular reason why the largest boards do not archive posts to an archive forum after so many posts or months?
There's very little functionality lost by searching the last 6 months separately from rest of time. I guarantee you that the commonly queried topics come up again and again now anyway, and an archive won't make people miss and repost questions anymore than they do now.
Like I said, just curious.