Depending on your iterator implementation (or, lackthere of), the functional boils down to your first example.

For example, Rust iterators are lazily evaluated with early-exits (when filtering data), thus it's your first form but as optimized as possible. OTOH python's map/filter/etc may very well return a full list each time, like with your intermediate. [EDIT] python returns generators, so it's sane.

I would say that any sane language allowing functional-style data manipulation will have them as fast as manual for-loops. (that's why Rust bugs you with .iter()/.collect())

Python map/filter/zip/etc. return generators, so they're lazily evaluated.

Clojure transducers as well.