Crystal's type inference is more powerful than most other languages. Maybe the most powerful of any fully statically typed language? (I would love to hear of any others that have taken the same approach as Crystal.) You can almost write Crystal like a dynamically typed language, and yet everything is still fully statically typed.

I believe you can speed up your compile times a bit by being explicit with annotations (which I often prefer anyway), but there's still a lot of overhead for the global type inference.