You are absolutely right. I have written Scheme interpreters in both languages. Compare https://github.com/nukata/little-scheme-in-ruby/blob/v0.3.0/...

  # Cons cell
  class Cell
    include Enumerable
    attr_reader :car
    attr_accessor :cdr

    def initialize(car, cdr)
      @car = car
      @cdr = cdr
    end

    # Yield car, cadr, caddr and so on, à la for-each in Scheme.
    def each
      j = self
      begin
        yield j.car
        j = j.cdr
      end while Cell === j
      j.nil? or raise ImproperListException, j
    end
  end # Cell

and https://github.com/nukata/little-scheme-in-crystal/blob/v0.2...

  # Cons cell
  class Cell < Obj
    include Enumerable(Val)

    getter car : Val            # Head part of the cell
    property cdr : Val          # Tail part of the cell

    def initialize(@car : Val, @cdr : Val)
    end

    # Yield car, cadr, caddr and so on, à la for-each in Scheme.
    def each
      j = self
      loop {
        yield j.as(Cell).car
        j = j.as(Cell).cdr
        break unless Cell === j
      }
      raise ImproperListException.new(j) unless j.nil?
    end
  end # Cell

and they will make the point clear. Ruby and Crystal are different languages, but you can translate your code from Ruby to Crystal line by line fairly easily.

For the performance boost, see https://github.com/nukata/little-scheme/tree/v1.3.0#performa... which shows times to solve 6-Queens on a meta-circular Scheme as follows:

* Crystal 0.34.0: crystal build --release scm.cr: 2.15 sec.

* Crystal 0.34.0: crystal scm.cr: 9.88 sec.

* Ruby 2.3.7: ruby scm.rb: 84.80 sec.

Compiled (and complex enough) Crystal code runs 39 times faster than the equivalent Ruby code in this case.