serial rapidio core throughput

hi

i built two boards using SOPC builder which using serial rapidio to transmit and receive,

features: 3.125 GBS, 1 serial lane, stratix 4, using st pass through and mm avalon.

i have build a testbench which transmit 20Mbits:

- avalon st - packet size = 244 bytes.

- avalon mm - burst size = 244 bytes.

i am getting throughput of ~1.4GBS, in both cases, less then half from the expected, can anyone tell me how can i increase the throughput?