Peter, I do know.
Is because the creation of CR items / revisions, which is heavily used in assessment. Also the db_transactions that locks the cr_item / revision tables are affecting as well.
Imagine you are serving an assessment with 20 questions, to 20 persons at the same time, and the creation of an new cr_item takes about 100 ms, that will end up in big delays to serve the request (if you increase the deadlock timeout) or you'll end having the deadlock. In this example, takes 2000 ms to create all the items of a given assessment, by 20 persons is about 40 seconds in total to create all the items requested at the same time, eventually you'll have deadlocks, and they will propagate among all the applications that might be using (creating / updating) cr_items / revisions at the same time.
Finally, intensive concurrent use of the CR is NOT scaling so far.
