I find that my system gets rather unresponsive when I redo fingerprints in digkam. System monitor shows high file reads in digikam and high writes in baloo_file, so I think the problem is that digikam changes images or the database and then baloo_file tries to re-index it.
Workaround is to disable baloo temporarily.
Or you also could disable baloo indexing in the akonadi folder (but the akonadi database should not be in the same folder as your image collection, otherwise you need to move it to another location).
*** This bug has been marked as a duplicate of bug 400704 ***