We've got two Fedora 27 servers that we use to host several virtual servers using libvirtd / Qemu / KVM that are experiencing the same issue. When freshly booted, network performance is great, pings are < 1ms, network transfer rates are as expected. After a couple of days the network performance degrades on the host servers and pings range anywhere from 10ms all the way up to over 100ms. Transfer rate via SCP is 500KB / sec instead of several MB / sec like it should be. This makes it impossible to manage the guest servers via virt-manager. However, network performance of the guest servers is good and does not suffer at all. Has any one had a similar issue or have an idea of where to start to troubleshoot this?
Please check tuned parameters for virtual host server, you can use tuned-adm or you can change directly on proc, hope all will be fine post that. Hope that will help.