Node.js 节点js请求外部https url极慢

我使用https.request方法调用远程api，使用agentkeepalive， 不过还是很慢。 keep-alive代理将利用未使用的代理，但是，如果第一次调用或套接字已关闭，它仍然非常慢。 有什么问题吗

   var HttpsAgent = require('agentkeepalive').HttpsAgent;

   var keepaliveAgent = new HttpsAgent({
        maxSockets: 100,
        maxFreeSockets: 10,
        timeout: 60000,
        keepAliveTimeout: 30000 // free socket keepalive for 30 seconds 
   });

   var options = {
        host: WECHAT_HOST,
        path: WECHAT_PREPAY_URL,
        method: "POST",
        headers: {
            "Content-Type": "application/xml",
            "Content-Length": requestXml.length
        },
        agent: keepaliveAgent
    };
    var request = require("https").request(options, function (res) {

        res.on("data", function (chunk) {
            ....
        });

        res.on("end", function () {
            ...
        });
    });

    request.on("error", function (err) {
        ...
    });
    //Logger.Write("Request ready");
    //console.log(requestXml.toString("utf-8"));
    request.write(requestXml);
    //Logger.Write("Data sent");
    request.end();//The problem is here, wait for about 12 seconds, if I invoke same request in firefox, the remote server response immediately.