Yes, this is a great example of a pillar of functional programming, pure functions. What is concerning is that your code example, like other pure function examples out there, implements a math operation a first grader can perform.
It's exceedingly difficult to find a pure functional example that operates on tangible domain-specific data of objects full of state.
It's not at all difficult (https://pandoc.org/). However, I'd hazard a guess that pandoc written in Rust or C (with idioms appropriate to those languages) would be much more performant.
I’m not as sure, several optimizations are only possible if we know that they can’t/won’t have side effects, and while the compiler can see it after inlining, having it part of the signature does help. Haskell is quite impressive on the performance front.
It's exceedingly difficult to find a pure functional example that operates on tangible domain-specific data of objects full of state.