GRE/gif/netgraph tunnel speed on 10Gbit channel

[Vitalij Satanivskij]

Hello.

Trying to build a tunnel between two machines with 10Gbit network (ixl0: <Intel(R) Ethernet Connection 700 Series PF Driver, Version - 1.9.9-k>)

I saw that the maximum speed of the tunnel is 1.6gbit for gre/gif and arround 544-600mbit for ng0 (netgraph tunnel via TCP)

Hardware is:

CPU: AMD EPYC 7251 8-Core Processor
    FreeBSD/SMP: 2 package(s) x 8 core(s) x 2 hardware threads
    real memory  = 137438953472 (131072 MB)

Also check same on 
CPU: Intel(R) Xeon(R) CPU E5-2660 v4 x2
CPU: AMD EPYC 7351 16-Core Processor x2 
Come intel E3-1230 and so on.

Version of system - 11.2-BETA2 FreeBSD 11.2-BETA2 #0 r334027
Also I'm test FreeBSD 11.1-PRERELEASE #6 r320593

Kernel GENERIC and CUSTOM (mostly cut off not used drivers from kernel) 

For testing I'm use iperf on pure 10Gbit chanel easy get 9.8-9.9Gbit's 
For tunnel's even try to check multiply instance of iperf (eg on different ports)


So question are - Is it normal speed for tunnel's. 
Which tuning I can try to speed up it. 

Thank you.