Another random thing I just remembered: if you're doing work in the VMs that is heavy on network I/O, try turning GRO off. We saw a big performance increase by doing this: https://github.com/contiv/volplugin/blob/master/Vagrantfile#...

One other thing: VirtualBox 5.1 is only supported by Vagrant 1.8.5 and newer, but 1.8.5 has a 100% showstopper bug where the SSH key inserted into the VM is not chmod'd correctly and OpenSSH refuses to look at it. Use 1.8.6 or newer instead.