WPF slow performance - many DataItem=null binding warnings
I have managed to get rid of all the binding errors!
I'm not sure why but adding the resources into Application.Resources instead of using UserControl.Resources has solved the Resource not found errors. This is far from an ideal solution (it would be much nicer to scope user control resources to the user control) but it works.
All of the other dataItem=null type errors were resolvable by providing fallback values in the various bindings.
This has resolved the performance problem that started this question so the answer to my original question is that fixing the binding errors does make a big difference to performance. Now I have fixed the bindings, my tree updates are almost instant instead of taking over a second :)
Many thanks for the help!
Jeremy