It doesn't hurt performance in any noticeable way. Your database plugs are passed into the actions. The plugs are designed to do only what you want them to do, so you can tune them to your hearts content. In fact, I would argue it is far easier to tune a query in Obvious than it is in a traditional ORM which hides the query from you.

This is done via simple dependency injection, which is pretty standard in many other languages outside of ruby.