If a given CPU can do X amount of work, with a C-based language you can maybe get 0.8X performance from that CPU.

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.