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.
Is this similar to that others have had?