I imported over 800 notes from Evernote to the Mac OS desktop Joplin, worked perfectly. Then I installed the iOS version and during sync it ALWAYS hangs at 230/500. I have one large notebook with over 600 notes. I have restarted the iOS application several times and it always dies at 230/500.
If you haven't already tried this, disable the screen timeout and leave the Joplin app in the foreground - i.e., start the sync, put the phone down and leave it alone. Initial sync on mobile will take a while.