I spent a good few hours trying to squeeze some extra performance out of our Vagrant / VirtualBox Development environments.
I was benchmarking different OSs, and virtual hardware configurations.
My guest OS is ubuntu 11. I was experimenting with adding virtual CPUs and suddenly noticed a massive network performance degradation. It seems changing the network adapter to Paravirtualized Network (virtio-net) wasn’t a good idea. Even the Intel PRO/1000 nics were problematic.
Best performance (with multiple CPUs) was PCnet-FAST III (Am79C973).
Hope this helps someone.