Some additional info for consideration. In addition to the forum (relying on mysql) exhibiting slowness, I also run Squirrel mail (no reliance on mysql) on the server.
When the forum is slow, access to squirrel mail is slow. This makes me think it's not a mysql problem, or better said, not just a mysql problem, but rather whatever the problem is is also bogging down the server, as the iotop shows.
OK, killing the server now to get some non-mysql/web-server iotops for comparison.