@Array#sum@ introduced in Ruby 2.4 is much faster than @Enumerable#inject@.

require 'benchmark/ips'

numbers = (1..10).to_a

Benchmark.ips do |x|
  x.report('inject') do
    numbers.inject(0, :+)
  end
  x.report('sum') do
    numbers.sum
  end
  x.compare!
end

Warming up --------------------------------------
              inject   169.646k i/100ms
                 sum   234.947k i/100ms
Calculating -------------------------------------
              inject      1.692M (± 1.7%) i/s -      8.482M in   5.013728s
                 sum      2.347M (± 0.9%) i/s -     11.747M in   5.006572s

Comparison:
                 sum:  2346592.2 i/s
              inject:  1692335.1 i/s - 1.39x  (± 0.00) slower

I have generated the attached patch with rubocop-performance 1.8.1 (Performance/Sum).