https://github.com/rails/rails/pull/42524

bench.rb

# frozen_string_literal: true

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", github: "rails/rails", branch: "main"
  # gem "rails", github: "ghiculescu/rails", branch: "has-many-build-perf-regression"
  gem "sqlite3"
  gem "benchmark-ips"
end

require "active_record"
require "active_support"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :authors, force: true do |t|
  end
  create_table :posts, force: true do |t|
    t.references :author
  end
end

class Author < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :author
end

author = Author.create!

posts = 100_000.times.map { |n| { author_id: author.id } }
Post.insert_all(posts)

author = Author.find(author.id)
author.posts.load_target

Benchmark.ips do |x|
  x.report("main branch") { author.posts.build }
  # x.report("my branch") { author.posts.build }
  x.compare!
end