I agree that the comment sounded pretty hostile, but I also agreed with the assessment that it might be better to avoid allocation in general. I know the code in the post is highly simplified, but you aren't exactly fully using "lazy iterators," at least in the post. refresh/load_hotels is still allocating new Hotels, each of which contains a one-million-element Vec. If you could reuse the existing self.hotels[id].prices Vec allocations, that might help, again at least in the example code.

On second glance, I guess this is what you're getting at with the ArcSwap comment in the post. It sounds like you really do want to reallocate the whole State and atomically swap it with the previous state, which would make reusing the prices Vec impossible, at least without keeping around two States and swapping between them.

Anyway, tone aside, I still think the comment had validity in a general optimization sense, even if not in this specific case.