Agreed -- I've found these pearls of received wisdom often crumble on non-trivial projects. "Premature optimization" can lead some to defer thinking about performance at all. "Profile and fix the hotspots" without simultaneously thinking about architecture can yield a mass of uniformly slow branchy code with no path forward. It's hard to appreciate until you've been there. However, I do have faith that mozilla folks have most certainly been there and that those lessons inform the rust design.