The Expression_Theme opens faster. It has less ControlTemplate elements and the initial load time is smaller. You can create custom theme based on the Expression theme and somewhat change the colors to look like the theme you want to use. The expression theme has no animations on the button, most of them are blank Borders while the Office_Black for example use several Borders and visual states have animations so it is slower.
the Telerik team
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to help you get started with RadControls for Silverlight