What are the conditions for tag quality to be other than Good?

The OPCUA server returns the Status code as Good, but the tag quality is Uncertain or bad.
Is it possible that the communication speed or other factors are affecting this?
What is the tag quality when the server response is slow?
If so, how slow does the response time have to be before it ceases to be Good?

You can see the reasons for a certain tag quality being picked on the Quality Codes and Overlays user manual page.