I believe this has been reported before in MDL-61517, but this problem renders Moodle unusable for course management if courses have 25 000+ participants. I figured we should have an open Moodle Tracker ticket about his issue.

It seems that when a gradable activity is added in such a course, a large amount of database queries are triggered. They seem to mainly be queries like this: UPDATE mdl_grade_grades SET aggregationstatus = 'unknown', aggregationweight = 0 WHERE itemid = '225' AND userid = '1123'

As a real-life example, a user tried to add twenty SCORM activities in a course with 45 000 participants, and each of them took 22 minutes. Adding them all would take a whole work day. It seems that the amount of activities already in the course affected the load time. In a course with only one SCORM it took 6 minutes, but in a course with tens of SCORMs it took 22 minutes.

Furthermore, in our 4.1.15 environment, this operation also caused Gateway Timeout errors: "The gateway did not receive a timely response from the upstream server or application." Interestingly, this also caused the sessions of some (but not all) other users to freeze for 10-15 minutes. I think this is the first time I have noticed that an operation started by one user results in sessions to be freezed for other users as well.