This is true with any any interaction with the DB, ORM or otherwise. Regardless of the layer of abstraction you choose to operate at you still need to understand the underlying complexity.

What Rails gives you is easy to use (and understand) abstractions that enable you to directly address performance issues.

Easy is highly contextual here, because none of this is trivial.

I think the real value in frameworks like rails and Django is that it makes it easier to collaborate. When you do it from scratch people inevitably write their own abstractions and then you can't share code so easily.