While a previous investigation yielded no meaningful results, another user confirmed the performance issue and provided a reproducer: https://www.mongodb.com/community/forums/t/mongoid-8-slow-insertions/225228/8

In their case, the slowdown is even more significant than in previously reported instances.

Use the provided code to reproduce the scenario and compare times in Mongoid 7.5.4 and 8.1.4.

Additionally, confirm that Mongoid 9 does not add to the problem to help decide whether we can release Mongoid 9 before addressing the regression.