You're looking at it in the wrong way. It's very easy to get started, but it's sort of "difficult" to get your head around how everything works, and where some of the "magic" comes from.

If you don't want to understand it, you can be on your way in 5 minutes. If you have a problem, you have the option of learning how it works. For example your point about databases is moot: were it not for rails, you'd still have to figure out how databases work. Now it became optional. If you do run into problems and need to learn how databases work, then you're no worse off than if you would have written the queries yourself, because then you'd still have to learn how the database works.

Most of the "compilation" of haml and erb and coffeescript is only once, after which it's cached and works just as fast as a static file. Still, there's some truth to the fact that it becomes a bit slower, but for most sites it's still well within bounds, and wouldn't be noticeable to the user unless you're running a 100 reqs per sec site.