[calorie]

Oh, didn't know you were referring to query cache and not vB cache. Not sure what you mean by 'scans the entire post table' though, as the query shouldn't be examining all the rows, but on a board of your size, it seems it's too many rows anyway. I think a separate table could have userid, forumid, and count, perhaps with forumid and count serialized, so at a minimum that'd be one row per user, which then could be a column in the user table instead, but there's still the updating to do. Not clear it's really worth it.