Often, especially with large file transfers, it seems that the shared folders facility in VirtualBox (running Ubuntu, but not sure that matters), running as a Windows guest, are very slow.
I added these lines
255.255.255.255 VBOXSVR #PRE 255.255.255.255 VBOXSRV #PRE
To this file:
And then restarted the VM. I saw a big improvement.