Here are some links on the Virtualizing of Treeviews. This concept has been around since 2005, but if you are starting on figuring out the XAML for both Silverlight and WPF, then the Bing search return (as well as the google searches) return some confusing links that can waste time.
I found that these links got me going quickly:
Bonus extra: LInk to a Virtualizing WrapPanel!:which I just found, haven’t tested yet:
That’s it for now.