my windows 7 VMs in virtualbox had TERRIBLE, TERRIBLE network performance…

… the prolem seemed to be ip6 + virtio drivers. if you want to use virtio drivers… disbale ipv6 in windows 7.

ip6 slows down network virtualbox virtio drivers

also try disable tcp auto tuning:

netsh interface tcp set global autotuning=disabled

To verify that it is dsabled:

netsh interface tcp show global