You seem to be missing the point lukashrb made regarding using Java data structures. Your claim was "It's inherent. Clojure will always be slow" which is demonstrable false as you can use interop with your host language (Java, JavaScript, Erlang) to use data structure that don't suffer from this. Wrap how you're using them in Clojure functions and the usage code for this is idiomatic while also having cache friendly data structures.
I understand the point, I'm not arguing that you can't do that or speed up your code by doing that.
Python is also slow, but you can still go fast (in some cases) by calling into C libraries like numpy - but the performance is coming from C, not Python. Python is still slow, it's just not an issue because you're only using it for plumbing.
But Clojure is immutable-by-default, that's the point of the language - it gives you various guarantees so you don't have to worry about mutability bubbling up from lower levels. In order to wrap your heavy path you have to go outside the Clojure structure and sacrifice that guarantee. You do lose structural integrity when you do that, even if you attempt to insulate your mutable structure. The system loses some provability.
Calling C from Python is very different from calling Java code from Clojure. Clojure always runs on it's host (Java/JavaScript), so calling functions via interop usually speeds up everything with zero overhead, compared to calling C code from Python which does introduce overhead.
Everything in Clojure comes from Java, it's unfair to compare it to "performance is coming from C, not Python" as Clojure works differently.
That wasn't really my point. There is still a translation process you have to apply to cross the boundary between immutable and native data structures though, and that has its own overhead.
In general I would agree, but a significant part of Clojure's appeal is that it's immutable by default, because that allows you to make certain assumptions about the codebase. Introducing mutable datastructures means you can no longer make those assumptions, so it potentially has much wider ramifications than e.g. calling into C code from Python.
