Loading large documents freezes the UI. Instead, the document could be imported and measured asynchronously in a background thread, and some progress indicator shown to the user.

Note: Image sources should be frozen when the document is loaded in a background thread.

Workaround: Following the steps:

- Import the document in a background thread using the specific format provider. 

- Freeze all images in the document using the methods in the attached file.

- Set the document to RadRichTextBox in the main thread using a dispatcher:

this.Dispatcher.Invoke(new Action(() =>
{
    this.radRichTextBox.Document = document;
}));