P.S. It's trivially easy to rebuild collections via Librarian Sync. I scrapped my current collections to see how the folder method works. Afterwards, just ran the two JSON-based methods to get it back to the desired state.
2245 Items
- Rebuild collections (from folders): 208 sec -> I think this created 600+ collections
- Delete all collections: 64 sec
- Rebuild collections (from calibre plugin): 40 sec -> 21 collections based on tags/custom columns
- Update collections (librarian): 54 sec -> 34 collections mostly based on author names (favorites)
As an aside, I thought I only had half the number of books on this Kindle. No wonder full library text search is slow.