I'm currently testing the RadGridView with Load On Demand behaviour and I'm having some issues.
I'm using Entity Framework and when grouping the GridView the rows within some groups appear twice. This happens sometime just after opening the groups but sometimes not until clicking on the row within the group. Then a second row appears within that group.
This only happens when using VirtualQueryableCollectionView as ItemsSource of the GridView. When using ObservableCollection everything is ok. It's also in both cases ok, when no grouping is used - also when removing the grouping after having the duplicated rows. The affected groups are mostly those displayed at the top of the gridview. When changing the sort order of the column before grouping the column, the groups with duplicated rows are at the bottom.
A second issue is the performance. When using ObservableCollection as ItemsSource the performance is ok. But when using VirtualQueryableCollectionView the performance is unacceptable slow. I can see the empty grid for about 2 seconds before it is populated with data - and this with a Loadsize of 50 items (in this example it's a bit faster because I reduced the columns from about 10 to 2). I noticed that when using GroupRenderMode="Nested" the performance is noticeable faster, but that isn't the recommended way because of performance issues when using nested grouping.