We are upgrading a client's site from 2.6 to 2.7 and have found slowness with the quiz upgrade (5-8 hours for upgrade). After looking at MySQL's slow query log, it looks like query from mysqli_native_moodle_database->get_columns is getting run over and over again. Going back to the quiz upgrade, there is a step where quiz is iterating over all quizzes and then updating/inserting records into quiz_slots table as needed. So, this is generating a lot of calls to get_columns which isn't cached because cache is disabled.
BONUS: Word on the street is that this same problem also slows down unit tests because MUC gets reset/purged after every test.