>When it comes to optimization, there's a useful adage: make it work, then make it fast.

When you need something to be fast, you better design it from the start to be fast. This is terrible advice for everything but some UI/web cases.

Speed is a feature. Not every feature can be just 'added' to existing code without changing most of it.

All of that is especially true for running simulations and the like. Whether these are fast is often determined by the architecture you decided on in the beginning.

Nowadays the world is full of libraries, frameworks, etc. that will never be as fast the competition, because they can't become fast without changing their APIs completely.