What version of the controls are you using, as your download history says nothing about RadControls for Silverlight or RadRichTextBox? Some information on your OS, Browser and implementation might also be helpful. Also, please share with us the hardware specifications of the machine you're testing on.
We are designing our control with performance in mind and we consider it already to be satisfactory even for large documents. There is also a similar discussion in this forum thread.
the Telerik team
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to help you get started with RadControls for Silverlight