I have an extremely large file sync (exported from ~twenty years on Evernote) and imported it into the Desktop version of Joplin just fine, but when I use Dropsync to sync the files it onto my Android phone file system via download and load it, the app lags for DAYS and will not load it all.
I can get a few notebooks loaded (maybe even half?) but not nearly all the notebooks and notes. I have a pretty recent phone that's faster than anything else I own, so I don't think it's the cellphone's fault.
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