This segment retransmitted issue can occur on any part of the path even in a LAN. For example, it can be an issue on the auto detect for duplex option on NIC. Sometimes it has to be explicitly fixed on your server and switch side to the right value. It can also be due to the NIC driver.
So it's difficult to tell you how to fix that issue for sure.
Ideally, you should not have segments retransmitted at all.
We recommend you to monitor your servers few minutes before and after your load testing in order to see the difference during the test.