My guess is that the source of the problem is a long-standing bug in macOS, where the sandbox imposes an unspecified limitation on the number of files that can be opened by a user.
This happens when individual files are opened in Mp3tag via File → Open… and also when files are dragged and dropped from another app. After what seems to be a “magic” number of files (around 3000 in my tests), the sandbox stops loading any more files. Any other operation that attempts to load files will then fail.
This is exactly what you observed when loading the text file to import also failed.
The details of this macOS bug are described here:
An example project that demonstrates the issue is available here:
To reproduce the issues you’re experiencing, I first loaded 300,000 files into Mp3tag and tried Convert → Text File - Tag, which worked without any problems. After that, I installed JRiver and imported the 300,000 files. I then dragged a subset of files to Mp3tag (starting with 150,000 and reducing the number with each iteration). This is when I reproduced the sandbox error, which made the app unusable in each failed session.
It's worth noting that the issue is triggered even before the list of files reaches Mp3tag, so it's not possible to workaround from inside of Mp3tag. Instead, loading of files should preferably happen on the folder-level, or via playlists — which brings me to the workaround for your initial task.
To work around the issue, I tried to find a method that doesn’t involve dragging and dropping from JRiver or opening individual files (since that would trigger the macOS sandbox limitation again). I came up with the playlist export I described in post #13, which works without any problems — even for the complete set of 300,000 files. There, Mp3tag can examine each of the file paths in the playlist, check the existing bookmark (the one on the root of the music library), and use it to open the file via sandboxed secure access.
I also reproduced the freeze when pasting large amounts of text using the text-box–based version of the converter. Contrary to my initial assumption in post #2, Mp3tag isn’t simply taking too long to split the lines. Instead, the macOS text control I’m using (NSTextField) is simply not suited for handling such large amounts of text. I’ll fix this in an upcoming release.