gistfile1.rb

thread_started_at = Time.now
thread_finished_at = nil
for i in 1..4 do
  Thread.start(i) do |t|
    puts "started thread #{t}"
    sleep 1
    puts "finished thread #{t}"
    thread_finished_at = Time.now if t == 4
  end
end

loop_started_at = Time.now
for i in 1..4 do
  puts "started loop #{i}"
  sleep 1
  puts "finished loop #{i}"
end
loop_finished_at = Time.now

puts "Thread finished in #{thread_finished_at - thread_started_at}"
puts "Loop finished in #{loop_finished_at - loop_started_at}"

gistfile2.sh-session

[23] pry(2.1.5-p273)> edit
started thread 4
started thread 2
started thread 3
started loop 1
started thread 1
finished thread 4finished thread 2
finished loop 1
started loop 2
finished thread 3
finished thread 1

finished loop 2
started loop 3
finished loop 3
started loop 4
finished loop 4
Thread finished in 1.001928
Loop finished in 4.003844

SES のメール送信処理とかで時間がかかるやつは CPU に待ち時間が発生しているのでスレッドにする方がよさそう。