There a mixed bag of small optimizations here:

  - Since underscores are converted to spaces and that we remove all leading underscores. By changing the order of operations we can leverage the faster `.lstrip!`.
  - Corrected the gsub regexp to no longer match empty strings (`+` vs `*`).
  - Modify the matched strings in place to save on allocations

```ruby
require 'benchmark/ips'
require 'active_support/all'

module ActiveSupport
  module Inflector
    def humanize2(lower_case_and_underscored_word, capitalize: true, keep_id_suffix: false)
      result = lower_case_and_underscored_word.to_s.dup

      inflections.humans.each { |(rule, replacement)| break if result.sub!(rule, replacement) }

      result.tr!("_", " ")
      result.lstrip!
      unless keep_id_suffix
        result.delete_suffix!(" id")
      end

      result.gsub!(/([a-z\d]+)/i) do |match|
        match.downcase!
        inflections.acronyms[match] || match
      end

      if capitalize
        result.sub!(/\A\w/) do |match|
          match.upcase!
          match
        end
      end

      result
    end
  end
end

%w(foo foo_bar_id ____foo_bar).each do |str|
  puts "== Comparing with #{str.inspect} (#{RUBY_VERSION}) =="
  unless ActiveSupport::Inflector.humanize(str) == ActiveSupport::Inflector.humanize2(str)
    raise "#{ActiveSupport::Inflector.humanize2(str)} != #{ActiveSupport::Inflector.humanize(str)}"
  end

  Benchmark.ips do |x|
    x.report('humanize') { ActiveSupport::Inflector.humanize(str) }
    x.report('humanize2') { ActiveSupport::Inflector.humanize2(str) }
    x.compare!
  end
  puts
end
```

```
== Comparing with "foo" (2.7.2) ==
Warming up --------------------------------------
            humanize    25.593k i/100ms
           humanize2    29.256k i/100ms
Calculating -------------------------------------
            humanize    263.989k (± 1.9%) i/s -      1.331M in   5.043110s
           humanize2    299.883k (± 2.2%) i/s -      1.521M in   5.075478s

Comparison:
           humanize2:   299882.5 i/s
            humanize:   263989.1 i/s - 1.14x  (± 0.00) slower

== Comparing with "foo_bar_id" (2.7.2) ==
Warming up --------------------------------------
            humanize    18.187k i/100ms
           humanize2    25.678k i/100ms
Calculating -------------------------------------
            humanize    183.702k (± 1.5%) i/s -    927.537k in   5.050326s
           humanize2    250.470k (± 2.5%) i/s -      1.258M in   5.026682s

Comparison:
           humanize2:   250469.6 i/s
            humanize:   183702.3 i/s - 1.36x  (± 0.00) slower

== Comparing with "____foo_bar" (2.7.2) ==
Warming up --------------------------------------
            humanize    18.577k i/100ms
           humanize2    24.686k i/100ms
Calculating -------------------------------------
            humanize    188.868k (± 1.5%) i/s -    947.427k in   5.017524s
           humanize2    255.650k (± 1.8%) i/s -      1.284M in   5.022833s

Comparison:
           humanize2:   255649.8 i/s
```