Q5.1.8: I like recursion, does it slow things down?

Yes. Emacs byte-compiler cannot do much to optimize recursion. But think well whether this is a real concern in Emacs. Much of the Emacs slowness comes from internal mechanisms such as redisplay, or from the fact that it is an interpreter.

Please try not to make your code much uglier to gain a very small speed gain. It’s not usually worth it.