Vagrant box take long time to access shared /vagrant folder I have a vagrant box running on windows 10 hyper-v

config.vm.box = "kmm/ubuntu-xenial64" #A standard Ubuntu 16.04 LTS (x64).
config.vm.provider "hyperv"  

Just after provisioning it, it seems to be running o.k.
but then if I exit and ssh back to it then trivial actions such as pressing tab after typing cd /vag would take tons of time (e.g 30 sec or more)
and after that a cd /vagrant worked o.k.
then ls -la worked o.k.
and another ls -la took something like 3 minutes or so.

During that time hyper-v is showing the virtual machine at 0% CPU.

Any suggestions on where I should look to try and figure out why is this happening?
It seems ther reason for this was that the SMB connections would disconnect.
using 

net config server /autodisconnect:-1


in a "run as administrator" command window fixed it.

as explained here:
https://www.vagrantup.com/docs/synced-folders/smb.html#preventing-idle-disconnects