OK I figured out what was the problem, and it doesn't come from you control, I think the CommandManager is guilty...
Sorry for the inconvenience.
I'm using RadControls for WPF Q3 2011 SP1 with these options:
It's still very slow when I navigate the nodes by arrow key.
It is slow too when I expand a node even with DropExpandDelay="00:00:00" and
Did I miss something ?