timhunt significantly improved this query in MDL-77147, but further evaluation identified that another significant jump in performance was possible. The new version was then tested by Tim with exciting, real world benefits:

We downgraded the DB size on our test Moodle instance to save money, and now 'my' version of the query ... well actually, there is a lot of variation, but in some cases it was taking ~75 seconds, and at other times ~12. Your version of the query seems to take ~5 or ~2 seconds (even with the join on question_bank_entries added back). So, probably worth another issue here.