title: There is no improvement when changing from sync to async handler
body: <p>I just compared sync and async version handler, and there is no improvement on 'response time' etc metrics, so how to debug in this situation?</p>
  
  <p>Handler</p>
  
  <pre><code>@gen.coroutine
  def get(self):
      calendar = Calendar()
      response = yield (calendar.events())
      self.write(response)
      self.finish()
  </code></pre>
  
  <p>Model</p>
  
  <pre><code>class Calendar(object):
      @return_future
      def events(self, callback=None):
          result = # MySQLDB operation
          callback(result)
  </code></pre>
  
  <p>Basic Information:</p>
  
  <pre><code>CPU: 1 core
  Dababase: MySQLDB
  </code></pre>