I maintain a 4 year old rails code base. I'm not going to be porting it to Crystal - but if I had an easy way to get ~5x speedup of the most performance critical parts of my code, I would jump at the opportunity. I'm glad someone is working on this

Crystal is not a Ruby. They're doing things that are intentionally different, and it's totally put me off considering it.

In practice 99% of the time plain old MRI is fast enough for the things I do. The rest of the time, resorting to RubyInline or a C-module has usually been more than enough.

E.g. the bulk of my code for my masters that involved heavy use of k-nearest neighbour implementations etc. for pre-processing image files for OCR was written in Ruby, with only ~2-3% then replaced with RubyInline C-code to get the speed needed.

I'd take that over Crystal any day, as I don't like the places where Crystal diverges.