[Vibraphon]

... But in case of the recently posted Firehouse Jazz Band Fake Book, its songs load extremely slow (a typical song may take about 20 seconds!). ...

That is exactly the book I used to test too. On my tablet it took some more than 20 seconds.
I did not test whether it would be faster using single PNG-files, but usually I use single PNG-files and they work much faster.

I too was confused about the file organisation. The whole PDF has been stored in a folder named to the first song name. All my other songs are stored in single folders, named to the song. Currently I don't know, how this should work together. Maybe I should create a special folder for CSV-books? I don't know.

...  I think I'd be letting a lot more people down if I didn't continue the Windows 10 development to focus on a feature that most people don't seem to need or care about. ...

The priority of developing features is up to you. This was just my idea.