That is: At least 90% of code is not CPU-limited. It's fast enough. So, as an initial approach, ignoring performance is fine.
The problem is that, if and when you run into real performance issues, you need to know some tools for dealing with them. If this book doesn't teach you those, well... it's a language introduction. It's not an advanced text. Maybe it would be the better for a warning or two, a paragraph here or there, but performance is (quite properly) not the point for the book.
I guess I mean to say that I really like this book, and would enjoy more from the author teaching fast Haskell, to supplement this book about simple Haskell.
I was pleasantly surprised recently when I noticed that this book has a discussion of efficient list construction [1]. I know it's a fairly basic idea, but the author brings it up purely for performance reasons. Given the introductory nature of this book I didn't expect that.
