On the other hand, Swift changes so frequently that book authors teaching the language can't keep up with Apple to write up-to-date books. By the time a book comes out covering version N, we're a couple months away from version N+1. In this situation I can't trust I will be able to buy a book, learn the language and become comfortable with it before there is a new version with major changes. It's a very moving ground.
As for performance, it would be cool if every single thing wasn't behind an atomic reference counter, making it slower than even garbage-collected Go: https://media.ccc.de/v/35c3-9670-safe_and_secure_drivers_in_... (the relevant part starts at 33:06).