Still note 4,since I want the fingerprint sensor working.

Upgrading the firmware did no good, still lagging.

This brought me to an uninstalling spree, berserk style.
I also reloaded Greenify and greenified the bulk of remaining apps, first of all Facebook.
This was good, since afterwards my phone is snappy again. I don't know if it's due to uninstalling or greenifying, but that's it.