The problem should be resolved with lib version 2015.3.1019.

The reason for the performance issues is the size of the ItemsSource for the combo column and the internal lookup logic.

A new IsLightweightModeEnabled property of GridViewComboBoxColumn will be introduced with R2 2016. Its default value is False. 
When set to True, the new lightened lookup logic is triggered, thus the performance of the column is significantly increased.

The issue seems to be a bug with UI Row Virtualization since once I set it to False, it did not occur any more. You can also refer to the documentation on UI Virtualization: http://docs.telerik.com/devtools/wpf/controls/radgridview/features/ui-virtualization.html

When the HorizontalContentAlignment of the GridViewCheckBoxColumn's CellStyle setting to Center, the checkboxes in the columns remain on the left side. If another theme like OfficeBlack is used, the checkboxes go on the center.

When loading the columns from a file the columns are read in the order they are found in the Columns collection of RadGridView upon saving them.
Setting the DisplayIndex triggers the internal grid logic that ensures the DisplayIndex of each column is correct. 
The combination of the above two operations can cause the grid to invalidate the DisplayIndex of columns while deserializing them and thus setting invalid values.
To ensure correct behaviour when deserializing sort the columns by their display order property before returning them in ProvideValue()

Provide an option to group a column which is based on an enum by the enum value, but display the DescriptionAttribute or DisplayNameAttribute if existing.
So many times you have sort/group a column by its value like 1,2,3 but display the Description like "Delivery", "Ready", "Pickup". Of course I could add a column to my class which presents the DescriptionAttribute, but then grouping/sorting is wrong: "Delivery", "Pickup", "Ready". It would by much easier if there is an option like "UseDescription". There is something similar with Teleriks "EnumDataSource" which IS using the Description or DisplayName so it should not be that hard to implement the same logic for RadGridView.

Implicit row's style should be not cleared when the SelectStyle method of RowStyleSelector returns null.

Available in LIB version 2016.3.1121, it will be also available in the 2017 R1 Release.

The exception is fixed. To evaluate the IsReadOnlyBinding of the column, we do not use the built-in binding mechanism but a custom one. This custom binding mechanism is designed to use as a source only the default one which is the current item of the ItemsSource of RadGridView. This is the reason that the scenario where the default source of the binding is changed is not supported by design. 

Available in LIB version 2017.1.213 , it will be also available in the R1 2017 SP1 Release.