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.