I am afraid RadGridView ( or any other ) DataGrid on the market would not be capable of handling such a heavy cell template. RadGridView is designed with much lighter templates in mind - e.g. TextBox,TextBlock, combo box etc. A control like a rich text box may often contain a visual tree with hundreds and even thousands of visuals that need to be taken care off while scrolling.
What I can recommend for your scenario would be:
1. Use a StackPanel to layout the multiple instances of the rich text box
2. Actually use the rich textbox only in edit mode.
In case you insist to use RadGridView for, please use the rich text box only in edit mode . This way you will have just a single instance of the heavy template as only a single cell may be in edit mode at a time.
All the best,
the Telerik team