I assume you have many notes, maybe 10k like I do. In that case Dropbox is way too slow. Also Filesystem on Android will not work because the android app cannot load a folder with 10k files, I had this exact same struggle with Syncthing. Use Joplin Cloud instead. For more info check How to survive with a large dataset on Joplin (10k notes, 20k resources, 10GB) - #3 by _vg