Why?

Total items to sync includes tracks, playlists, folders and playlist tracks I think!

Everything is synced in batches to the limit of what’s possible. Basically if you’ve reimported an entire library, for example, everything will have been refreshed and therefore it will sync.

I also noticed a similar problem: tracks are duplicated when syncing via Google, and when I uploaded a backup, the library was duplicated.

Decided to give selective sync another go. This is going on for half an hour without progress and counting now. How can I see, what is happening there?

Okay, after 2 and a half hours it is at 3 % now. that is way too slow.

And I still don't see it doing anything?

Sync can complete in seconds. I suspect there is a problem here. What happens if you do Settings/Reset Tools “Resync This Library”? Or if that’s greyed-out try restarting MIXO first.