LevelDB is what we chose. It is not SQL… no SQL library would come even close to performing here. SQL databases normally implement something like LevelDB internally for their indexes.
Lots of software uses LevelDB – Autodesk AutoCAD and Google Chrome come to mind.
The other thing to note is that you have so many files because we save your artist photos and album artwork as individual files. We use a hashed deep directory structure to avoid having directories with many thousands of files, which can cause performance issues.
This is not an area we are changing, or even entertaining a change.
If your backup software is breaking, I suggest you use better software for your backups. If it’s just slow, well, that’s the reality of having all your data local.