Accessing ENV variables from ENV is slower than a constant in Ruby

env_variable_test.rb

class TestClass
  def test
    var = ENV['TEST_VAR']
    var == 1
  end
end
require 'benchmark'
Benchmark.bm do |x|
  x.report { 1000000.times { TestClass.new.test } }
end

 => [#<Benchmark::Tms:0x007fe26c956810 @label="", @real=0.6151940660020045, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.6100000000000001, @total=0.6100000000000001>]
 => [#<Benchmark::Tms:0x007f803b02e258 @label="", @real=0.6114436259995273, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.6000000000000001, @total=0.6000000000000001>]
=> [#<Benchmark::Tms:0x007ffba2961f18 @label="", @real=0.6205215830013913, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.6100000000000001, @total=0.6100000000000001>]


module MyModule
  ENV_VALUE = ENV['TEST_VAR']
  class TestClass
    def test
      var = MyModule::ENV_VALUE
      var == 1
    end
  end
end
require 'benchmark'
Benchmark.bm do |x|
  x.report { 1000000.times { MyModule::TestClass.new.test } }
end


 => [#<Benchmark::Tms:0x007f893b8ced78 @label="", @real=0.25701763500183006, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.26, @total=0.26>]
 => [#<Benchmark::Tms:0x007f9672890160 @label="", @real=0.2614059769985033, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.26, @total=0.26>]
 => [#<Benchmark::Tms:0x007fc2031bb7b0 @label="", @real=0.25079641200136393, @cstime=0.0, @cutime=0.0, @stime=0.0, @utime=0.25, @total=0.25>]
   
   
   
   # Just by adding one more zero (1 million to 10 million) times on both, we get almost 3 seconds difference
    => [#<Benchmark::Tms:0x007fde1a0d7938 @label="", @real=5.9996224630012875, @cstime=0.0, @cutime=0.0, @stime=0.019999999999999997, @utime=5.96, @total=5.9799999999999995>]
   vs
    => [#<Benchmark::Tms:0x007fde1b0aed98 @label="", @real=2.5135619059983583, @cstime=0.0, @cutime=0.0, @stime=0.020000000000000004, @utime=2.4799999999999995, @total=2.4999999999999996>]
      

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