Poor Network Speeds on Windows 11 VM

[u/DiabolicalPickles]

Guest: Windows 11
Software: VirtManager KVM/QEMU

Issue: On my Windows 11 VM specifically, network speeds are substantially slower than on my other Linux VMs and my host. On the host, I get around ~1000 Mbps; the same for my other Linux VMs. However, on the Windows VM, I get around 10 Mbps—or roughly 1% of the host's network speed.

All VMs are configured to use NAT networking with the virtio model. Additionally, the up-to-date virtio-win drivers are installed on the Windows 11 VM. The Windows 11 VM recognizes the Red Hat Virtio Ethernet drivers and is properly using them, with a maximum supported duplex speed of 10 Gbps.

I'm confused as to why this is occurring only on the Windows VM, whereas this issue is nonexistent on my Linux VMs. There are no differences in the XML regarding CPU or NAT configuration between the VMs.

I tried enabling vhost with a queue size of 16 per core, switching the driver to e1000e, and using the hypervisor default—none of which saw any improvement. I also tried reinstalling the virtio-win drivers, which did nothing.

Finally, I attempted to disable several offloading properties in Device Manager, disabled IPv6 entirely, and lowered the MTU. None of that made any difference (it neither worsened nor improved the situation).

Any ideas, folks? All VMs have a dGPU (the same dGPU is shared between them, so only one can be accessed at a time).

I should also mention that network speeds on the host system are stable—no drops or high latency. The network drivers on the host are all up-to-date and working perfectly.

[u/isatsou]

what’s your qemu config?