Our setup uses Cloud-front as a load-balancer/reverse proxy in front of our Moodle. Because of this, it is very important to have define('NO_OUTPUT_BUFFERING', true); set on for pages which take a long time to process. Two particuarly notable examples of that are:

• Re-grading a quiz with many attempts.
• Computing the statistics for a large quiz.

(Comlex question types like STACK exacerbate that.)

Unfortunately, NO_OUTPUT_BUFFERING has to be set before doing require_once config.php, so we cannot do this selectively. So, I think we need to set it at the top of mod/quiz/report.php.