CX5 25G card total bandwidth problem

Hi, I’ve a CX5-512A-ACAT card.
I notice this card share 25G bandwidth to dual port. This looks like a hardware limitation.
I did the following test.

Server A port 1 <-25G-> server B port 1
Server A port 2 <-25G-> server C port 1

Server A open two iperf server.
Server B open iperf client and connect to server A
Server C open iperf client and connect to Server A

In Server A console, showing 12 - 13G throughput in each iperf server console.

  1. PCIE BW satify? check by lspci -s -vvv
    2.iperf, or say TCP/IP stack handle by kernel, need tuning to get max performance