The class \backup_questions_structure_step found in backup/moodle2/backup_stepslib.php has a DB query for getting the tag instances associated with a question:

         $tag->set_source_sql("SELECT t.id, ti.contextid, t.name, t.rawname

                              FROM {tag} t

                              JOIN {tag_instance} ti ON ti.tagid = t.id

                              WHERE ti.itemid = ?

                              AND ti.itemtype = 'question'", array(backup::VAR_PARENTID));

This query seems to be costly because it's not using a better index. You can test when running EXPLAIN:

After some digging, the taglookup index is used if the component is specified: