So here's the story:
Running Windows Server in QEMU with basically default options on Ubuntu results in horrible upload speeds for whatever reason (you can see this when running Google's speed test, or any other different kind of speed test which isn't your usual Flash-based). These are the command line options I am using:
-net nic -net user,hostfwd=tcp::3389-:3389
However, the way Proxmox runs QEMU results in better speeds because it is using a "bridge". I don't fully understand how it works, but I know the key lies in the /etc/network/interfaces configuration.
How do I make and use a bridge in QEMU to provide my VM internet access with Ubuntu 14.04?
I only plan to forward a handful of ports to the VM, including RDP (3389) of course. I am using an OVH dedi, so it has IPv6 and IPv4 on eth0.