In order to increase the scrolling performance you could:
increase the Height of the rows (GridViewRow)
increase the FontSize of the cells (GridViewCell)
increase the Width of the columns
Please note that the bigger visual elements you have, the faster the scrolling speed will be.
As of Q3 2012 we have introduced a new property of the RadGridView GroupRenderMode. The default mode is Nested, and the new one is Flat. When you set the Flat mode, the GridView will render rows one below the other. This leads to a very good perfromance when the grid is grouped on several levels and has a lot of data.