There is a similar discussion here.
Can you try pre-loading the page you are trying to load?
Also the RadNavigation framework is left only for bakward compatibility with older Telerik applications. It is recommended to use the navigation framework that is now available in the Silverlight framework.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items.