I noticed your thread on phpbb.com: https://www.phpbb.com/community/viewtop ... &t=2220886
You don't give much information to go on. If you're on a shared host, all bets are off. The CPU getting pegged could be due to anyone else on the same server. If you're on a dedicated server, I would start by getting some statistics from mysqltuner:https://raw.github.com/major/MySQLTuner ... qltuner.pl
If you want to eliminate phpBB-SEO as a possible cause, install the base phpbb.com download on the same server but different directory (e.g., /forums/ for the phpBB-SEO install and /forums3012/ for the "virgin" phpBB install). Modify the config.php to point to the same database. That will also make it easier for you to ask for help from phpbb.com since you will have no mods or forks involved.
My assumption is that your 100% CPU problem has nothing to do with your software, i.e., your host is sharing the CPU and it's overloaded.