out.cr

    gbf 775.64M (  1.29ns) (± 1.99%)  0 B/op        fastest
builtin 231.94M (  4.31ns) (± 1.15%)  0 B/op   3.34× slower

test.cr

require "benchmark"

input = Array(Int32).new(1000000)
1000000.times do
  input << rand(100000000)
end

Benchmark.ips do |x|
  x.report("gbf") { get_index_of_max(input) }
  x.report("builtin") { input.index(input.max) }
end

def get_index_of_max(array)
  max = 0
  prev = 0
  array.each_with_index do |x, y|
    if x > prev
      max = y
      prev = x
    end
  end
  max
end