It's a much bigger problem because it's not just "wp_dropdown_categories" that is affected. I see slow queries also for:
/wp-admin/edit-tags.php?action=edit&taxonomy=category&tag_ID=1461&post_type=post
tag_id should point to a category with many children that also has sub children.
4.2.1 is really slow and it hurts site performance in a way (timeout erros) that I had to roll back to 4.1x
Changes in taxonomy.php (esp.: get_terms() and _get_term_children()) should be checked carefully. Maybe unit tests were still okay but bad performance impact was not noticed while trying to improve things.