Would you please try configuring RadGridView with GroupRenderMode="Flat"? As explained in our documentation, the Flat mode was introduced with Q2 2013 as a completely new logic for group rendering. The result was faster scrolling and very good performance, but it fixed a lot of other issues reproducible with the old Nested mode.
As the column virtualization is enabled by default, when scrolling is done all the columns have to be realized. In order to make the scrolling way faster, you can try setting ScrollMode="Deffered".
Let me know if this helps.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.