I'm wondering: are there any RXRS compliant scheme implementations that are competitively fast? say comparable to js or lua, or even C/rust? If not, is the reason coming from scheme itself, or is it more a lack of effort in this direction?
I would say the reason it beats Chicken and Gambit is that it has a Scheme-specific compiler to machine code that has been worked on for a long time with performance in mind.
Dybvig's talk on Macro Writers' Bill of Rights explains some of the optimizations Chez does: https://www.youtube.com/watch?v=LIEX3tUliHw (Why macro writers? Because the compiler will handle superfluous generated code so macros doesn't have to)
See also The Development of Chez Scheme for the history of it: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.72....
I'd guess if you're sure your `fib` function handles only integers, you should get a lot of easy inlining/specialization at your disposition.
Chez instead provides typed/unchecked arithmetic functions, like fx+, flround and so on. In my experience, those sometimes make a big impact on performance(though realistically in some of those cases it would be better to ask C to do it for you).
Racket similarly makes it possible to do either.
Edwin Brady (Idris 2 creator and main author) has mentioned a few times that Chez is surprisingly and impressively fast.