This is an expected behavior. When having Extended selection, a custom selection handler is created that manipulates the information about the first/last selected indices, selected item/items. When there is a SortDescriptor, a sort function is created and if we need to find the index of an item, we must take that sort info into account. 
Searching the item will pass through the logic of a binary search and verify whether the item in the middle is the same as the one we search for. And since we have a sorting function, we work with the value of the property used in it. In this case, all values are the same and the binary search returns the item in the middle (as both have value "1"). Considering that this is not the one we want, another search has to be made. And this slows the performance. 
The way to go is either to set SelectionMode to "Multiple" or disable sorting of columns for which there is no benefit of sorting (such with equal values for each item).

Hi,

We are going to investigate the problem, however we cannot commit to a certain time frame when it will be fixed. You can follow this item in the feedback portal. That way you will get a notification every time its status get changed.