Google pub/sub client high latency inside java web servlet in appengine

I try this example Github to use Google pub/sub java example to push messages inside app-engine servlet, however my logs shows latency up to 11 seconds when my servers are overloaded.

Any idea about this issue?

public class SendMessageServlet extends HttpServlet {

@Override
public final void doPost(final HttpServletRequest req,
                         final HttpServletResponse resp)
        throws IOException {
    Pubsub client = PubsubUtils.getClient();
    String message = req.getParameter("message");
    if (!"".equals(message)) {
        String fullTopicName = String.format("projects/%s/topics/%s",
                PubsubUtils.getProjectId(),
                PubsubUtils.getAppTopicName());
        PubsubMessage pubsubMessage = new PubsubMessage();
        pubsubMessage.encodeData(message.getBytes("UTF-8"));
        PublishRequest publishRequest = new PublishRequest();
        publishRequest.setMessages(ImmutableList.of(pubsubMessage));

        client.projects().topics()
                .publish(fullTopicName, publishRequest)
                .execute();
    }
    resp.setStatus(HttpServletResponse.SC_NO_CONTENT);
    resp.getWriter().close();
}

}