In the Library Module the Sort Order is stored per folder and saved in the DB, the AgFolderContent table (see attached). The default SO seems to be "Capture Time" despite the value of the AgLibrary_defaultSortType in the Lightroom 6 Preferences.agprefs file. The value stored in the preferences is the value of the currently selected folder's SO on exit. But this value does not seem to have any bearing on what SO a new folder will use, so it seems to be useless. I was hoping that I could set the preference file to a value and have it use it for new folders, but that was not the case.
The Import dialog Sort Order is stored in the preferences file - AgImport_defaultSortOrder. And is updated with the value of the last completed Import session. This seems to stick and not be saved anywhere else.
I wasn't aware of this feature until I started looking into this tonight. I think I would like to have the same sort order for everything - File Name in my case. But am always careful about what I ask for. SO by Folder may have some utility, although I have never used any of the others. My file naming convention is better than Capture Time (YYYYMMDD_HHMMSS_IMGNUM) with the image number sequencing files within the same second, so File Name is my go to SO.
This "By Folder SO" is a feature that I suspect contributes to the slow image count display that I complained about in Ron's discussion. But I still think it could be much quicker.