Paulc,
When you say you had Network/shared data off, did you untick the "Get Flights From RadarBox Network" at the bottom of the Network list and untick "Share Flight Data" in the Preferences settings.
You have to untick "Share Flight Data" every time you run RadarBox without an internet setting, as this is not remembered when you shut down and the default setting is "On".
Running RB without a good Internet connection with "Share Flight Data" selected, can cause RB to run slowly.