Loading performance was due to a bug in the XMl reading. Now it is lightning fast. Turned out I updated screen while loading.
I still am using a bit time as I load a Tree document at end, but as indicated before I can fix this.
The last performance issue is that I have an unexpected 2-3 sec delay as I create special symbols – this smells like a bug as well.
This is good because I was getting worried that WPF was not up for the task as number of diagrams and symbols grew, but it actually looks good.