I used it in production for a client last yr and i compared that with a go implementation. The concurrency support of crystal on a 40 Core machine beast the socks off golang. Crystal both maxes out all the cores to a high degree, on file io and overall speed of execution. The time taken to do the same task Crystal; beats go by a significant margin while being so easy to write. And go is a easy language to write but Crystal is so much more easy to read because of Ruby syntax inheritance.

We use it to generate sitemaps with >30 million entries. Ported a Rails task that took ~9 hours to Crystal, using a library with the same API as the Ruby gem we had used. Essentially a copy-paste-tweak that took a couple hours of effort. Crystal version now runs in prod in about 30 minutes.

I believe Kagi (https://kagi.com/) uses a lot of it.

Have been using it in production for years for small projects. But also have a current, larger project in it that is going well. A few years ago I built a high throughput (20k shortens/s) link shortener for work, in Go. I later rewrote the main path in Crystal to test it for performance. It was a bit faster than the Go version (22k/s) on the same infrastructure… but running on a single core! We did not finish/ ship it, but it was revealing.

If you're running or using Invidious [1] - a YouTube proxy service - you're running or using Crystal.

[1] https://github.com/iv-org/invidious

Using it extensively in production at https://heiioncall.com/ . It’s a joy to work with. Fast to write AND fast to run. :)