We have a RadBook control with Left and Right PageTemplates defined.
In both templates, there is some content defined along with a RadBusyIndicator used to display a loading message during the load of the actual controls in the template.
The problem is, that the RadBusyIndicator instances are not released from the memory and it seems that they even prevent the actual controls to be garbage collected.

Workaround:
The issue is caused by a limitation of the Silverlight framework and the Telerik Window8 theme in which in order for the the progress bar animation to be smooth additional  EaseIn has been set to its KeyFrames. In order to resolve this issue all you need to do is remove the EaseIn from the Indeterminate VisualState of the Style which targets RadProgressBar. You can find attached a sample project which shows the described workaround.