Other than the amount of text in a document, the type of content can also influence the performance of RadRichTextBox. You can check the Performance article
for information on the known causes for such behavior and if you send us the document, we will look into the cause of this.
Don't hesitate to ask if you have any additional questions.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.