There are two different scrolling modes.
In a standard scrolling mode, the grid loads the entire summary data at once. This operation may affect grid performance as the loaded data may contain many summary values.
In a virtual scrolling mode, the grid loads a summary value at runtime when it gets into a field of vision. Once a summary cell is out of the field of vision, it is removed from the grid. This behavior allows an end-user to scroll through large amounts of grid records without noticeable lags.