Adding new books is intensive.
A simple MDE (metadata edit) happens fairly fast EXCEPT when you change someting that affects multiple books.
Changing the case of a Tag, is one of those
science fiction -> Science Fiction dirties all records with the tag
(the tag mapper is case insensitive, so adding a book with the wrong case, flips all usage.
Use an 'intake' library, to mash things into your styl;e, the simply right click: Copy to Library <optional delete from this one. A
safe 'Move' >