I'm pretty sure this doesn't measure what it claims. It's a mostly sequential flow with very little meaningful concurrent work going on. I would imagine a for loop would put all of those to shame. I'm not sure why anyone would optimize their runtime for what's basically yet another process ring benchmark but I know it's gotten slower over time on a couple of the measured cases because it's just not a real example and making this fast trades off other things like good scheduler balancing.

Also check out Pony if you want something were spawn and send become basically free. It'll run pretty close to for-loop speed for the number adding which should be a baseline calculated in all implementations.