You can't make a Ruby program that uses fixnums-only. You also can't make a Crystal program that's dynamic like Ruby. Interestingly, though, when the Crystal implementation is modified to use bignums, the Ruby implementation is over twice as fast while still being dynamic.

What I get from this is that Crystal's static compiler is slower than Ruby's dynamically dispatched interpreter, and Crystal's bignums are so slow you'll need to think hard about whether you want a 50x speed boost or correct arithmetic in all cases.

I think Crystal is a good concept, but it's only version 0.34.0. Every implementation is bad at version <<1.0. Ruby <<1.0 didn't have good performance, either. I'm sure Crystal will be great by the time it gets to 2.6.5, too.