"Elixir is blazing fast" it says, so I follow the link and notice that "blazing fast" apparently means 10x the speed of Ruby. I don't think that phrase means what they think it means.
Elixir and Erlang are bad at number crunching, they aren't "fast" at computations like C is. However, because of the amazing piece of tech that is the BEAM, problems that are best solved through multiple processes/actors, concurrency, and distribution appear to be "blazing" fast because the two languages are very good at that type of stuff. Hence the Phoenix web framework being called "blazing fast"—the BEAM is great for servers.
With elixir you might get 0.4X performance. From that perspective Elixir looks slow.
But you can deploy 10 servers and thus get 4X performance overall, with essentially no cost for Elixir. On the other hand to make distributed system in C you will spend 10X the development time.
So Elixir is fast in the sense that you get a distributed system much faster, and you can get more overall performance by scaling across multiple CPUs.
(And I'm kinda avoiding that on a quad core CPU your C program is really going to only use one core out of the box while elixir will use all four.)
These ratios are just examples, of course.
https://github.com/elixir-lang/elixir/wiki/Interoperability-...
I think that's a pretty major factor to avoid -- multicore processors are the immediate/commercial foreseeable future of CPUs, so its not inconsequential that Erlang/Elixir allow you to be able to use cheap processes in order take full advantage of this