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Intel 82599ES "slow" SR-IOV speeds

Hey everyone!

I am running Arch Linux with an Intel X520 (82599ES).
I am also running a Windows 10 VM with Qemu/KVM/OVMF.

I have an SR-IOV VF passed through to the VM for networking.
The Windows driver is v. 2.1.174.0 and the Linux driver is the 5.1.0-k ixgbe kernel driver.

I am connecting to a CentOS server through a Mikrotik CSS326 switch. The server also has an X520 NIC and a 10Gb connection to the switch.

Using a Samba share that runs on a FusionIO PCIe SSD on the server, I can comfortably saturate 10Gb on Arch. However, using the same share on the Win 10 VM I can only reach 300MB/s, i.e. exactly 2.5Gbps.

It feels as if my VM is limited to 2.5Gbps instead of being able to use all 10Gb.

Any ideas?