The book was just published before 3.4.0 released. This is significant because R 3.4.0 introduced the JIT compiler by default (if I remember correctly, I added managed to sneak a footnote into the print version). This means that when you write a script, all functions are automatically compiled.
However, packages aren't byte compiled by default (only user code). As I mentioned in my book, you might only get a tiny speed up (say 20%), but for a single line change, that still seems worthwhile. I suspect that a future version of R will implement this, but for now, it's up to you.
For info, I've been byte compiling all external packages for over three years, with no issues.
It seems from R 3.5.0 packages will byte compiled by default