Information from the gtk-list gives at hand that invoking this function
under certain circumstances has some undesired side effects. I propose
adding the following text to the manual section on GtkSortable or in the FAQ:

------------
Calling gtk_tree_sortable_set_sort_column_id() on a certain GtkTreeStore or
GtkListStore will turn on sorting for this list store. This will slow down
performance on subsequent data additions from O(n) to O(n^2). There is no
way to turn sorting off again. If you repeatedly add several (say
thousands) of rows to a tree or list store, this will cause a noticable delay.

Therefore, if you desire large, sorted lists and trees, the procedure to
follow when filling the store anew is:

1. Create a new list store with gtk_list_store_new()
2. Replace it for the one currently connected to your view with
gtk_tree_view_set_model()
3. Invoke g_object_unref() on the old store and destroy it.
4. Fill the new store with some thousand entries.
5. Turn sorting back on using gtk_tree_sortable_set_sort_column_id()