bench.rb

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report("reverse_dependencies_name") do
    # inspect or else it will benchmark just the creation of sql query
    # and not the execution
    Version.reverse_dependencies('rails').inspect
  end

  x.report("reverse_dependencies_id") do
    rails = Rubygem.find_by(name: 'rails')
    Version.reverse_dependencies_by_id(rails.id).inspect
  end

  x.compare!
end

version.rb

#...

  def self.reverse_dependencies(name)
    joins(dependencies: :rubygem)
      .indexed
      .where(rubygems: { name: name })
  end

  def self.reverse_dependencies_by_id(id)
    joins(:dependencies)
      .indexed
      .where(dependencies: { rubygem_id: id } )
  end
#...