In my consulting experience, I'm shocked if I find that inefficient queries aren't the root cause of poor performance. In fact, thinking about it, I don't think it has ever happened. I always look at the DB layer first because it's virtually guaranteed that someone wrote a "SELECT * FROM MassiveTable" and added it to the common header code used by every page.
Matches my experience too, both consulting and in-house as the person who'd be the first to even consider looking at the database query logs or running "explain" on queries.
A reason for ORMs is that a lot of developers fear the database (EDIT: not the only reason, to be clear; I love and use ORMs). A result of ORMs is that a lot of developers think they can avoid understanding the database.
