Everything that isn't SQL tuned to your environment forces you to sacrifice performance at some point. At some point it does an inefficient join. How can you not go back and fix performance issues is you don't know SQL? (It isn't like you need to fix it in C)
SQLAlchemy makes dropping down to SQL where necessary incredibly easy. There where necessary you can tune the queries, but where not necessary you can let the ORM create them for you.
This gives you a lot of flexibility and power, but as zzzeek mentioned, you need to know SQL to understand how what the ORM is inefficient and to be able to replace it as necessary while still letting the ORM do what it is really good at.
