If you have a long running or complicated task that you want to distribute across multiple processes, you can break it into small segments and use DelayedJob to distribute the workload. You can send a lot of email quickly using the methodology below (and a lot of DJ workers).

doing_complicated_things.rb

User.find_in_batches do |user_batch|
  do_something_complicated(user_batch)
end

def do_something_complicated(users)
  users.each do
    user.something_complicated
  end
end

handle_asynchronously :do_something_complicated

# https://tech.bellycard.com/blog/load-balancing-for-speed-with-delayed-job/