dns is the cause of many seemingly unrelated problems. some services (like sshd) do reverse dns lookups on connecting ips. a misconfigured dns server somewhere (or improper delegation) along the path can make this initial connection take up to 30 seconds while waiting for dns timeouts. it may look like an extremely slow/busy server, but in reality it's just sitting there doing nothing waiting for a dns reply.

tools like dnstracer (http://www.mavetju.org/unix/dnstracer.php) and dig are very useful for diagnosing these issues, but you really need to understand the fundamentals of how dns and delegation work.