ixgbe:about latency

Hi,

I used l2fwd to test ixgbe PMD's latency (packet length is 64 bytes) found an interesting thing that latency is about 22us when tx bits rate is 4M and latency is 103us when tx bits rate is 5M.

Who can tell me why?Is it a bug?

Thank you very much!

Regards,
Haifeng