bench.rb

require 'benchmark/ips'
require 'active_support/all'

class CVarBase
  mattr_accessor :legacy_connection_handling, instance_writer: false, default: true

  def self.connection_handlers
    unless legacy_connection_handling
      raise NotImplementedError, "The new connection handling does not support accessing multiple connection handlers."
    end

    @@connection_handlers ||= {}
  end

  def self.connection_handlers=(handlers)
    unless legacy_connection_handling
      raise NotImplementedError, "The new connection handling does not setting support multiple connection handlers."
    end

    @@connection_handlers = handlers
  end
end

class CVarChild < CVarBase
end

class RefactorBase
  module Config
    extend self
    attr_accessor :legacy_connection_handling
    @legacy_connection_handling = true

    attr_accessor :connection_handlers
    @connection_handlers = {}
  end

  def self.legacy_connection_handling
    Config.legacy_connection_handling
  end

  def self.legacy_connection_handling=(enabled)
    if (Config.legacy_connection_handling = enabled)
      Config.connection_handlers ||= {}
    else
      Config.connection_handlers = false
    end
  end

  def self.connection_handlers
    Config.connection_handlers or raise NotImplementedError, "The new connection handling does not setting support multiple connection handlers."
  end

  def self.connection_handlers=(handlers)
    unless legacy_connection_handling
      raise NotImplementedError, "The new connection handling does not setting support multiple connection handlers."
    end

    Config.connection_handlers = handlers
  end
end

class RefactorChild < RefactorBase
end

Benchmark.ips do |x|
  x.report('cvar') { CVarBase.connection_handlers }
  x.report('const_ivar') { RefactorBase.connection_handlers }
  x.compare!
end

results.txt

Warming up --------------------------------------
                cvar   538.684k i/100ms
          const_ivar     1.463M i/100ms
Calculating -------------------------------------
                cvar      5.298M (± 1.6%) i/s -     26.934M in   5.085454s
          const_ivar     14.427M (± 2.3%) i/s -     73.140M in   5.072354s

Comparison:
          const_ivar: 14427114.5 i/s
                cvar:  5297607.2 i/s - 2.72x  (± 0.00) slower