[capink]

I made a quick test with set_metadata and it was slower than single field edit even for one book. It compounded the problem and calls Last Modified plugin multiple times per field per book.

from calibre_plugins.action_chains.actions.base import ChainAction


class Test(ChainAction):

    name = 'Test set_metadata'
    support_scopes = True

    def run(self, gui, settings, chain):
        book_ids = chain.scope().get_book_ids()
        
        db = gui.current_db
        cache = db.new_api
        
        if len(book_ids) == 0:
            return

        for book_id in book_ids:
            #mi = cache.get_proxy_metadata(book_id)
            mi = cache.get_metadata(book_id)
            mi.set('#test1', 'text')
            cache.set_metadata(book_id, mi)

Edit: It seems set_metadata() is calling set_field() for every field, even if the field value was not changed.