No idea, but it doesn't surprise me.
It depends primarily on the size of the list, (less so) on the size of the library and how many books were series before.
RL has to search for all books with that series, then spin through all the books in the list setting that column, then spin through any books that had that series before that
aren't on the list clearing that column, then commit the changes.
I don't see any obvious inefficiencies, unless db.set_custom() has started started doing commits too.