Javascript WebSocket服务器未立即接收消息

Node.JS服务器无法快速接收消息

服务器代码：

var WebSocketServer = require("ws").Server
var http = require("http")
var express = require("express")
var app = express()
var port = process.env.PORT || 5000

app.use(express.static(__dirname + "/"))

var server = http.createServer(app)
server.listen(port)

console.log("http server listening on %d", port)

var wss = new WebSocketServer({server: server})
console.log("websocket server created")

wss.on("connection", function(ws) {
  var id = setInterval(function() {
    ws.send(JSON.stringify(new Date()), function() {  })
  }, 1000)

  console.log("websocket connection open")
  ws.on("message", function(msg){
    ws.send(msg);
  })
  ws.on("close", function() {
    console.log("websocket connection close")
    clearInterval(id)
  })
})

要连接的代码：

var z = new WebSocket("ws://appname.herokuapp.com/");
z.onopen = function(){ console.log("opened"); };
// ... after opened message seen in console, the following
// were typed directly into the console, a few seconds apart
z.send("H");
z.send("Hello World");
z.send("Hello World asdf");

在第三次发送完成之前（即使是在其他发送之后一分钟），服务器不会响应

H

或

Hello World

；所有三个回答都是同时作出的。如何使服务器及时响应

（使用WireShark，我检查了发送是否确实完成；它们是）

编辑：每次响应前的请求量/时间都不同