After some days I'm updating this thread.

Essentially the VM now works for a day, allowing clients to connect and do their job, but it must be rebooted every day as after hours it seems to slow down and stop responding on the network.

You can successfully connect to the vSphere console, see the VM screen, use it but at a snail pace, several seconds interlaps between a click and the opening of a window.

It's Win2012R2 twin works without any need of intervention to day.

So, if anybody has information RE: Win2012 VM very slow or unresponsive, I'd be glad to read it, thanks.