Terrible performance on D820 laptop if the network cable is unplugged

[damian]

But it works fine when the home network is plugged in.

I'm glad I went to dual boot rather then completely over to Ubuntu as 
the machine is almost unusable when I'm out and about if using Ubuntu. 
Then it's suddenly completely fine once plugged into the home network. 
Can anyone help me to find out what's going on here?

The machine takes a LONG time to boot and then is very sluggish when 
it's not networked.

Looking forward to any help in solving this

Thanks
Damian