The bug #278 may be not related to my issue. I face slow NIC issue either using Realtek, e1000 or Virtio, but after upgrade to 2.2-30 I got very fast NIC, nearly 70 Mbps (my Switch is 100 Mbps).
Unfortunately they are not related. I've been following a mailing list for qemu that dietmar has been discussing this issue in. This isn't a Proxmox related issue. There is no fix yet, just a workaround, but the work around unfortunately involves disabling vhost which triggers the issue.