I've started the migration using the Lightroom Plugin. It seems to be going very slowly. I don't see resource constraints (e.g., CPU utilization < 25%; there remains available physical memory; disk I/O is very low) and my files are all "by reference." It has been running for about 14 hours and it has processed 9% of about 60,000 images. I've noticed it reports it is Building Previews for images that are already in the Lightroom Library, which seems odd. Other than intermittent beeps it is progressing.