As reported here https://moodle.org/mod/forum/discuss.php?d=400012#p1613678
adding a lesson to a course is forcing a regrading of the whole course.

Our installation of Moodle has more than 100,000 users and has been running for about 10 years (3.10 is our current install) so the whole regrading thing is causing the script to systematically timeout, regardless how much memory or time we throw at PHP.

We are thereby unable to add new lessons.