I am using a custom row style and row template for RadGridView together with AlternationCount=2. When sorting the columns or moving the scrollbar, performance is extremely choppy. I have uploaded a sample project that demonstrates the bug:
To reproduce, just start the project and use the sorting features in the gridview and/or drag the scroll bar.
On line 471 of MainPage.xaml you will find some more comments about the issue.
I am hoping that this can be fixed in an internal build soon.