https://github.com/rails/rails/pull/29999

update_record.rb

# frozen_string_literal: true

gem "bundler", "< 1.16"

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

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

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

  gem "rails", github: "rails/rails"
  gem "sqlite3"
  gem "benchmark-ips"
end

require "active_record"
require "benchmark/ips"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")

ActiveRecord::Schema.define do
  create_table :foos do |t|
    t.integer :value, default: 0
  end
end

class Foo < ActiveRecord::Base
end

class SubFoo < Foo
end

foo = SubFoo.create!

Benchmark.ips do |x|
  x.report("_update_record") { foo.value += 1; foo.save! }
end

Before:

Warming up --------------------------------------
      _update_record   150.000  i/100ms
Calculating -------------------------------------
      _update_record      1.548k (±12.3%) i/s -      7.650k in   5.042603s

After:

Warming up --------------------------------------
      _update_record   201.000  i/100ms
Calculating -------------------------------------
      _update_record      2.002k (±12.8%) i/s -      9.849k in   5.027681s

30% faster for STI classes.