I have a VPS server which contains just two web sites. For some reason, the wp-comments-post.php file on one of these is using over 50% of the server's entire CPU usage. Does anyone know why this is? I'm presuming this must be due to the site receiving a lot of spam but I have no idea how I can reduce the spam we're receiving. How would it be possible to reduce this CPU usage?

Any suggestions are welcomed!