Problem in benchmark_tx.py and benchmark_rx.py

I am trying to run benchmark_xx.py and modified the benchmark_tx.py.
My benchmark_tx.py transmits file once whether error occurs or not.
In this point, I could see odd results:

  1. Receive rate is more and more better when I transmit big file.
  2. If there is sleep code after pkt_send function then receive rate is
    changed based on sleep seconds.

I think there are some reasons for these results(either HW or SW
problems).