Exchange Server 2013 SP1 - Internal Email Flow slowness

Hi Fellows,

I have a brand new implementation of Exchange Server 2013 SP1 with three mailbox servers and 3 CAS servers.
I am facing a considerable slow transportation of email internally. although there are just 3 mailboxes on the infrastructure at the moment.

When a user sends email to himself or other two accounts, delivery takes from 15 seconds to 35 seconds to be delivered.
Experience is same even if the email is sent from OWA, ActiveSync or Outlook.

Header analysis shows the message exchange between mailbox servers is taking time. any clue?
Decreasing Tarpit interval on Mailbox server receive connectors will be helpful?

Network communication, Storage performance, Server performance are all as good as we love to.

Thanks.

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J.A