If the files are in a Virtualbox shared folder from the host OS, that performance is to be expected. Just google "Virtualbox shared folder performance."[1] If you're using Vagrant to manage the VM it's easy to work around this by switching the file share mechanism to nfs. If you're not using vagrant, you can do this manually too, or just move the files to a non-shared folder inside the VM.

[1] Scroll down to the graph at http://mitchellh.com/comparing-filesystem-performance-in-vir... to get an idea of how bad this is.