In practice 'good iteration velocity' is often dependent on using a language/stack that you and/or your team is comfortable with and proficient in.

When one can choose between multiple equally-mastered stacks, then choosing the most appropriate/performant one is perhaps a good choice. But even then there are other things to consider.

For example, I worked for a startup where my ideal choice would've been a performant stack that I'm pretty good with (Elixir/Phoenix), but we ended up going for Python/Django because the founders felt that 1) it would be easier to find more programmers locally, and 2) apparently some investors do care about this. Both seemed like valid reasons to me.

Every maintained project is a continuous Ship of Theseus rewrite, but yes, if there's a language your team is more productive at writing correct code in and that provides better performance, there's an easy choice to make. That's very often not the case, though, and as well as team it's often project scale dependent, and projects often start at a scale where the calculus is very different from where it might be years down the road.