Amavisd-new will invoke SpamAssassin + ClamAV for spam/virus scanning, it will take most system resource.
You can try to decrease concurrent Amavisd process to reduce its resource usage.
*) set '$max_servers = 1;' in /etc/amavis/conf.d/50-user
*) update line 'smtp-amavis ...' in /etc/postfix/master.cf', replace the number by '1' (same as '$max_servers' in Amavisd).
smtp-amavis ... 1 ...*) Restart Postfix service first, then restart Amavisd service.