fast.rb

def index
  # use whatever AR scopes you want to make your sql query
  relation = Post.find(1).comments.recent.where(author: params[:author_id])
  
  # Then convert to sql to get an array of arrays back - way faster than
  # full-on ActiveRecord objects
  result = ActiveRecord::Base.connection.execute(relation.to_sql).map do |row|
    {
      author_id: row[0],
      something: row[1],
      something_else: row[2]
    }
  end
  
  # yajl is a fast JSON serializer written in C
  render json: Yajl::Encoder.encode(result)
end