To reproduce:

Add a RadRichTextBox to a form and use the following code to add pages:

            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < 1000; i++)
            {
                sb.Append("a word ");
            }

            for (int i = 0; i < 10; i++)
            {
                this.radRichTextEditor1.Insert(string.Copy(sb.ToString()));
                this.radRichTextEditor1.InsertPageBreak();
            }

You will notice the poor performance upon scrolling or modifying the text.

Resolution: 
This issue is addressed in the new version of the control - RadRichTextEditor. Please use the new control instead the RadRichTextBox.