Scrolling through text of richtextbox is slow and not smooth

When I scroll through the text of my rich text box, it doest scroll fast and smoothly. It works perfectly fine for files of size up to 1 mb, but it gets very slow if I use file in the range of 10mb. The richtextbox I am using is on the form named subform. I am doing subform.TopLevel = false and putting inside a panel on Mainform. From Mainform using object of Subform I am calling subform.richtextbox.Loadfile(filepath). 

 

If I directly run the Subform and load file like this.richtextbox.Loadfile(filepath), the scrolling is working perfectly fine for same file.

 

Please note , the loading of file itself takes much time, which is not a concern for me righ now. I will look into that, once I solve this issue. I want my scroll and all UI operation to work like it works in Notepad and Notepad++ without freezing.

 

Quick reply is very much appreciated. 

Thanks