Being that old, maybe it got web drive-by hijacked and is now cryptomining on your phone just through browsing. I browse very little on mine as I don't particularly trust android browsers.
I'll second wiping it with a factory reset to see if it helps, it'll get rid of any scurvy you picked up along the way. Put back your apps slowly and see what bogs it down.
The most compelling reason to upgrade these days is getting security updates, and samsung is pretty bad about it even on their latest phones, non-existent on yours that old. Maybe see if you can flash a newer rom os like LineageOS on there that is more current.