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Let me get straight to the question. In this scenario we are using static class RadRichTextBox which is global for all RadRichTextBoxes that you use in your application. In order to make this work as expected you need to set the property before the RadRichTextBox is initialized. If you need to set the property only for particular RadRichTextBox you can use this code snippet:
.radRichTextBox.TextRenderingMode = Telerik.Windows.Documents.UI.TextBlocks.TextBlockRenderingMode.TextBlockWithPropertyCaching;
Be aware that this property needs to be set after RadRichTextBox is initialized.
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