The agent should attempt 18 times to connect to the ePO repository (6 IP, 6 DNS, 6 NetBIOS). If the agent is able to resolve to the server, it should take 240 seconds for the socket to timeout, this could explain the long delay.
As for the HTTP anf FTP fall backs, the agent only has the DNS name, so there are fewer attempts.
You could contact support and submit an FMR for more granular control of the updateing mechanism.
Hope that helps.