Unfortunately, I can't pinpoint the issue due to insufficient information. It is normal that RadRichTextBox will work a bit slower with bigger documents. Can you please give us some more info on the document you are using or even better, send it to us so we can reproduce the problem?
Also, I suggest you debug your application and observe its performance. It is possible that something else is causing the issue, especially since the performance is better in our online demos.
It will be best if you open a support ticket and attach a project so we can investigate further. Thanks in advance!
the Telerik team