UBB.threads is a simple PHP script, it uses standard MySQL queries to fetch data from the database, with a huge database comes lag from having to sift through a large amount of content to retrieve the right posts. As we update for each new version of PHP we're striving to refine old database queries for clarity and speed (remember that this was initially a PHP4 script based on a Perl script).
You could try trimming out old data, or upgrade resources on the machine, there's not much else one can do other than leveraging a Content Delivery Network (CDN) to block bad bot traffic...
