Comment by munk-a

I have some serious doubts that an ORM can tune queries as well as a human due to the fact that the ORM lacks one key piece of information that both me (and the database planning a query) can leverage - table statistics. An ORM can produce a query that will behave well in the best general circumstances, but as soon as you get into topics like subquery performance fencing ORMs simply have no ability to compute optimality on the fly. One specific example I've seen is where multiple paths exist through the database to transit from one fact to another with one path being more strictly optimal and the other path having more associations that may be needed - this can effect the join strategies you want to use so if your ORM is anything more than "I'll essentially tell you the SQL but in a weird syntax" then there's a good chance it'll chose the wrong path.