test.rb

require 'active_record'

ActiveRecord::Base.establish_connection 'sqlite3:///:memory:'
ActiveRecord::Base.connection.instance_eval do
  create_table :items
  create_table :events do |t|
    t.belongs_to :item
  end
  create_table :event_people do |t|
    t.belongs_to :event
    t.belongs_to :person
  end
  create_table :people
end

class Item < ActiveRecord::Base
  has_many :events
  has_many :event_people, :through => :events
  has_many :people, :through => :events
end

class Event < ActiveRecord::Base
  belongs_to :item
  has_many :event_people
  has_many :people, :through => :event_people
end

class EventPerson < ActiveRecord::Base
  belongs_to :event
  belongs_to :person
end

class Person < ActiveRecord::Base
end

puts "Populating Database"
1000.times { Item.create }

Item.all.each_with_index do |item|
  event = Event.new
  event.people.build
  event.people.build
  item.events << event
end

require 'benchmark'
Benchmark.benchmark do |x|
  x.report('limit 100 has_many') {
    Item.preload(:events).limit(100).order(:id).to_a
  }
  x.report('limit 100 has_many_through') {
    Item.preload(:event_people).limit(100).order(:id).to_a
  }
  x.report('limit 100 double has_many_through') {
    Item.preload(:people).limit(100).order(:id).to_a
  }
  x.report('limit 1000 has_many') {
    Item.preload(:events).limit(1000).order(:id).to_a
  }
  x.report('limit 1000 has_many_through') {
    Item.preload(:event_people).limit(1000).order(:id).to_a
  }
  x.report('limit 1000 double has_many_through') {
    Item.preload(:people).limit(1000).order(:id).to_a
  }
end