Notes added in Calibre browser viewer won't sync to content library

Issue: Highlights or notes added in a browser on iOS or Android take a long time to sync to Calibre library. They don't appear right away (mine take 15-20 minutes and having to restart Calibre library, etc) before the annotations actually sync.

What I tried:
* Enabled content server
* Enabled the setting: Preferences > Sharing over the net > Advanced > Allow unauthenticated local connections to make changes.
* Accessed a book in Firefox on Android, added notes, they do save locally.
* Pressed "Sync" in the Calibre browser app.
* Checked the book in Calibre source library---it failed to sync the notes.
* Tried a couple rounds of reloading book in browser, restarting Calibre, re-syncing the book from browser. Eventually the notes synced but it took a really long time. Not clear what causes this latency.

System details:
* Calibre 5.14 for Windows 10 x64
* Samsung Galaxy phone, Android 11, Firefox browser