It doesn't appear to be multi-threaded and it takes forever.
It loaded the list of files between the two drives which took a bit, but not unreasonable since both drives have about 1.9M files on them. However the comparison is still running and has been running for about 24 hours and it's only at 89%.
Options selected:
Mode: One way copy
Compare : date (different) or size
Synchronize sub-folders
Ignore seconds when comparing by timestamp
Sync left-to-right
Hide unaffected files
Automatically shrink panel