Both SpamAssassin + ClamAV are content-based spam/virus scanner, they eat much system resource.
You can try disable them for OUTGOING emails first, track whether they use much system resource:
http://iredmail.org/wiki/index.php?titl … oing.Mails
if you're not satisfied, try completely disable them:
http://iredmail.org/wiki/index.php?titl … amAssassin