> Both NIC supports the speed of 1000Mb/s

How do you know? Your kernel or NIC config might be limiting you.

iperf, snfnettest, or etherate will show you the maximum possible bandwidth at the TCP or IP layer.
If it's under 700 then you know to focus on the NIC and OS. If it's above 900 then the
problem is in your nginx or your test workload.