It's funny how it starts out with "immutability is really fast and GCs are soo good" and ends up with "rewriting everything in unmanaged code made it 50x faster". Similar how "Ruby and Python interpreters are slow but webapps are IO bound anyway so it doesn't matter" to "how can I get this to handle more than x req/sec, can we get a JIT to speed up our dog slow backend".
asciinema prototyped and worked out their design, and discovered it needed more performance. And then it got it. Sounds like a success story to me.