Forum Configuration (Editing a Forum's Settings) -> Default date cut off for topic display - Setting this to anything other than "Show All" actually causes it to perform worse than it has to. The reason is an extra query is run when you are pruning topics by date to gather the valid data for page links and such. Unless you have a good reason, just leave this on "Show All".
You can try running this query to accomplish this quickly.
UPDATE ibf_forums SET prune=100 WHERE 1
UPDATE ibf_forums SET prune=100 WHERE parent > 0