As of Q3 2012 we have introduced a new property of the RadGridView - GroupRenderMode. It has two options:
Nested Mode: It is the default one and it will nest GridViewGroupRows into one another when you have grouping on many levels. This may lead to a poor performance when the grid is grouped on several levels and has a lot of data. The visual element representing the grouped row is GridViewGroupRow.
Flat Mode: This mode represents our new virtualization mechanism with a completely new item container generator. It simply renders rows one below the other. This leads to a very good performance when the grid is grouped on several levels and has a lot of data. The visual element representing the grouped row is GroupHeaderRow.
For compatibility reasons, the old render mode is also preserved, but it is recomended to use the new one.
Please note that when you use the Flat Mode, you should work with GroupHeaderRow, not GridViewGroupRow.