What a difference disabling MPTT makes!
We have been having trouble with (a) database migration time, and (b) the time it takes to create batches of Works. Disabling MPTT at the project module level reduces the migration time from numerous hours to 4 minutes[1]! It also improves batch creation.
Of course, if removing MPTT breaks things (as opposed to just removing an optimization), then that is a problem. If it doesn’t break anything, then I am very interested in disabling it, with the knowledge that I could re-enable it in the future using _rebuild_tree (though that would be unlikely).
[1] To be fair, there is also another optimization that is cutting out some time, but most of the change is MPTT.