What is the expected communication speed between MTAs sending mail

My 2 MTAs work fine, but I found, that it takes quite a lot of time for
big messages to be transferred from one MTA to the other and that's
much more time, than the time required to send that message via the
Internet Agent to another mail server. The messages are transferred at
a data rate of max 500kbit/sec over a 1Gbit/sec connection regardless
of the surrounding traffic. Small messages are transferred almost
instantly, big messages with attachments can take some minutes due to
the slow transfer speed even if there is absolute nothing else to do on
the two involved servers.

The MTAs run on OES 11 SP2 servers and the MTAs are on GW 2014 SP1.

W. Prindl