Indeed, but I think even that advice, with context, is pretty debatable. Obviously one should prioritize critical sections, but completely ignoring those "small

Indeed, but I think even that advice, with context, is pretty debatable. Obviously one should prioritize critical sections, but completely ignoring those "small efficiencies" is certainly a big part of how we got to where we are today in software performance. A 10% jump in performance is huge; whether that comes from a single 10% jump, or a hundred 0.1% jumps - it's exactly the same! So referencing something in particular from Unreal Engine, they actually created a caching system for converting between a quaternion and a rotator (euler rotation)! Obviously that sort of conversion isn't going to, in a million years, be even close to a bottleneck. That conversion is quite cheap on modern hardware, and so that caching system probably only gives the engine one of those 0.1% boosts in performance. But there are literally thousands of these "small efficiencies" spread all throughout the code. And it yields a final product that runs dramatically better than comparable engines.