RadRichTextBox supports two different rendering mechanisms. This topic explains what you can expect from them and how you can switch between the two options.
The default rendering mechanism of RadRichTextBox uses the Direct2D technology and the FormattedText class. Although it is faster, it can sometimes lead to unexpected behavior when rendering the contents of a document.
The legacy option is the TextBlockWithPropertyCaching rendering mechanism which uses draws a single TextBlock per line. The advantage here is that this mechanism prevents some rendering glitches which might occur while using the Direct2D technology.
Something to note is that the Direct2D option is faster. You might experience some performance diminishments in the rendering when using the TextBlockWithPropertyCaching.
There are two options allowing you to control the rendering mode of RadRichTextBox. The first one is through the DefaultTextRenderingMode property of RadRichTextBox. This is a static property and affects all instances of the control. There is also an instance property called TextRenderingMode which changes the value only for the current instance.
Example 1: Change the rendering mode for the current instance of RadRichTextBox
this.radRichTextBox.TextRenderingMode = TextBlockRenderingMode.TextBlockWithPropertyCaching;