array_process_benchmark.rb

require 'set'
require 'benchmark/ips'

def process_old(list)
  list.join(" ").split.uniq
end

def process_set(list)
  unique = Set.new
  list.each {|str| unique.merge(str.split) }
  unique
end

def process_hash(list)
  unique = {}
  list.each do |str|
    str.split.each {|word| unique[word] = true unless unique.key?(word) }
  end
  unique.keys
end

words = %w[ alpha bravo charlie delta echo foxtrot golf hotel india juliet
  kilo lima mike november oscar papa quebec romeo sierra tango uniform
  victor whiskey xray yankee zulu ]

# Returns a string of between 2 and 12 words chosen randomly from the
# given array
def rand_words_str(words)
  words.sample(rand 2..12).join(" ")
end

list = Array.new(1000) { rand_words_str(words) }

Benchmark.ips do |x|
  x.report("Old") do
    res1 = process_old(list)
  end

  x.report("Set") do
    res2 = process_set(list)
  end

  x.report("Hash") do
    res3 = process_hash(list)
  end

  x.compare!
end

result.txt

Calculating -------------------------------------
                 Old        98 i/100ms
                 Set        34 i/100ms
                Hash        58 i/100ms
-------------------------------------------------
                 Old      995.9 (±1.4%) i/s -       4998 in   5.019369s
                 Set      343.5 (±1.7%) i/s -       1734 in   5.049819s
                Hash      587.1 (±1.7%) i/s -       2958 in   5.039485s

Comparison:
                 Old:      995.9 i/s
                Hash:      587.1 i/s - 1.70x slower
                 Set:      343.5 i/s - 2.90x slower

Aren't you missing |times| for the 'old' method?

So I cloned your repo and removed the |times| from the first method above and re-ran the test using your code. Here's the results as per my comment on SO.

Calculating -------------------------------------
                 New       152 i/100ms
                 Old       388 i/100ms
-------------------------------------------------
                 New     1529.7 (±1.2%) i/s -       7752 in   5.068305s
                 Old     3910.0 (±1.2%) i/s -      19788 in   5.061618s

Comparison:
                 Old:     3910.0 i/s
                 New:     1529.7 i/s - 2.56x slower