Run the attached sample project, paste the path to the TestTemplates folder in the text box and click Start. You will notice the that the illustrated time in the labels gets bigger and bigger.

Workaround: this is not a complete solution but is reduces the load time:

radDiagram1.Clear(); 
            radDiagram1.DiagramElement.FindDescendant<Telerik.WinControls.UI.Diagrams.Primitives.BackgroundPageGrid>().FindDescendant<Telerik.WinControls.UI.Diagrams.Canvas>().Children.Clear();