Your assumption that filter/map/reduce is necessarily slower than a carefully handcrafted loop is wrong, though. Rust supports these features as well and the performance is equivalent.

Also countless real-world production environments run on Python, Ruby, JS etc, all of which are significantly slower than a compiled FP program using filter & map.

> FP languages are dead-set on "immutability" which simply means creating copies of objects

Incorrect. The compiler can make it mutable for better performance, and that gives you the best of both worlds: immutability where a fallible human is involved, and mutability where it matters.