I have server which is equipped with I210 NIC and has 11.2-STABLE installed.
With default config and MTU 9000 this server can not send faster than ~670Mbit/s on network without any other load (tested with iperf3).
If I set "sysctl dev.igb.0.fc" to 0, it easily sends 990-995Mbit/s at same network to same client.
Other problem is, that after mtu change with "ifconfig igb mtu ..." speed drops again, but "sysctl dev.igb.0.fc" is 0 still. To turn off flow control again it is needed to flip "sysctl dev.igb.0.fc" to non-zero and to zero again.