These are "guests" browsing your forum? Be sure to check IP addresses (with whois) to see if these are various search engines that have finally found your forum and are simply indexing it.
If it's just one IP pounding on your forum, it could be a DoS attack, or it could be a bad search engine spider (I think Baidu has a bad reputation for things like this). Some spiders seem to get hung up on pages with forms (e.g., login required) and just sit there burning CPU cycles. Either way, you could put an IP block in your .htaccess file to keep the offender away:
# block by IP address
# port scans on PC
deny from 22.214.171.124 <== change
allow from all
If it's really a severe pounding, you may want to inform your host, so they can put their own block in "further up the food chain" and decrease load on their internal networks. If it's a DDoS attack (many IP addresses attacking, too many for you to individually ban), your host may need to do something.