I am not certain whether these problems are with Avalon or the app. But here goes:
Build a release version and run it out of VS (just to be sure there's no/less debug code).
Now open up task manager and the cpu usage graph. Now run the app and put the continous update mode.
Watch CPU usage graph rise for a minute or so.
You can try minimize the app and the cpu usage stays same.
I have created some performance session with not too big result:
The System.Windows.Media.RotateTransform.set_Angle(float64) function takes longest within the UpdateClockHands function.
I have inserted a line into the UpdateClockHands function which outputs the "n" TimeSpan variable values. UpdateClockHands called normally by 50 msecs as given to the DispatchTimer.Interval property.
I'm not sure, it's a bug or not.