kTLS slower than non-kTLS

Hi,
My linux is: Linux linuxsws015 4.15.0-041500-generic #201802011154 SMP Thu Feb 1 11:55:45 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux

If follow your Readme and saw SSL_sslwrite show better perfomance (0.66s) while send/send_file took 0.77s.
I thought kTLS better but this practice showed the opposite.
Please advise.
Thanks,
Dinh.
