virtualbox.org

IMHO it is faster if you do not bridge inside the host as you are doing here.

auto br0
    iface br0 inet dhcp
    bridge_ports eth0 vbox0

I don't have this in my interfaces file. Try commenting this out and see if it speeds up. If not then put it back.

I don't do it that way. Bummer. looks like i'm stuck with 100mb speed on 1000mb network. terrible

Extremely Slow Network Speeds On Windows Guests