I think the best way forward is to write a reproducer for this, so the devs can easily verify and profile the problem.

You could take a look at my two PRs demonstrating a different performance issue and adjust them to insert more data and use SUBSELECT:

https://github.com/hibernate/hibernate-orm/pull/5741

https://github.com/hibernate/hibernate-orm/pull/6749

Having such reproducers will greatly improve our chance of getting this fixed.