Could you please explain the significance of the link between recipient schema and the xtkFolder schema in the structure of recipient as it impacts the query performance? Also, it has been noted that many of other schemas doesn't have a link towards xtkFolder schema. Any specific reasons to it?
The only impact would be viewing recipients in the UI, where it would check folder permissions.
For the non-folder schemas (no folder-id col), I'd guess the reasons are historical; You can make your own folder links for them by setting the view's query on the folder, either to an existing col or naming convention, or a new folder-id col.