> However, I do think that one should not arrive at the belief that this kind of
  > optimisation is warranted everywhere and that code simplicity can also be a goal

Definitively not warranted everywhere, especial if premature and at such detail, but core utils like `cat` and `yes` are IMO prime examples of where such optimizations are warranted:

- They're in use daily on a huge amount of setups, even small benefits add up much more than in some niche tool.

- They got a clear and small feature set that won't change anytime soon, so there won't be much code churn and thus maintenance effort will be relatively low.

  > Similarly, there is the NSFW comparison between Plan 9’s cat and GNU coreutils’
  > cat [3].
  >
  > [3]: http://9front.org/img/longcat.png

IMO that isn't an exactly fair comparison though, as the difference is not only in optimizations but a lot in boilerplate license/copyright comments and in features like line numbering, modes for showing (non-printable) special characters or whitespace and option parsing for said features.

Strip all that out, and you got (eye balled) 1/3 of that, and 200 lines for an extremely fast core util is really not much nor hard to maintain, as it won't get any new features soon anyway.