Why aren't you caching your forum as static pages for users who aren't logged in at the very least? E.g., rebuild the cache every x time as a cron task, but even then, every page load shouldn't be incurring database overhead if someone isn't even logged in. Equally, you can force a rebuild of cache for x relevant pages when someone posts a new thread or comment.

Otherwise, if someone alt+clicks a bunch of a category's threads as they look interesting then you're going to have a bad time.