If you need to work with big sets (in my case 1,000 records) ActiveRecord is slow. And so is calling to_json to that collection.

This is my workaround to transform a ~3000ms request into ~300ms:

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def index
  # before
  # @users = User.where('some condition').sort('some order')
  # render :json => @users

  # after
  @users = ActiveRecord::Base.connection.select_all(User.where('some condition').sort('some order').to_sql)
  render :text => @users
end

Also, in your front-end you need to transform the response into valid json (I’m usign jQuery):

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$.get('/users', function(users) {
  users = jQuery.parseJSON(users.replace(/=>/g, ':').replace(/nil/g, 'null'));
  // Do the rest
});

NOTE: I understand that this is not suitable for most cases, like for a public API, given that the output is not valid JSON. Also it has a lot of limitations, like it wont deserialize your serialized_attributes, you cannot use the to_json options like, :methods, :include, etc.