ESP8266 as serial wifi module - expected performance?

I'm using a puck.js v2 + esp8266 (ESP-01) module on Serial1 to serve files stored on an SD-card.

While I understand that this will never be a super fast file transfer method, I wonder what kind of speed could be expected? I'm currently getting between 600-700 bytes/s, and increasing the UART speed from 115200->460800 didn't really make a difference so I suspect that the UART is not the bottleneck.

Example code:

var wifi;

function mountSDCard() {
  SPI2.setup({mosi:D30, miso:D28, sck:D29, baud: 8000000});
  E.connectSDCard(SPI2, D31);
  try {
    console.log(require("fs").readdirSync());
  } catch(error){
    console.log(error);
  }
  E.setFlags({unsyncFiles:1})
}

function onInit() {
  Bluetooth.setConsole(true);
  mountSDCard();

  Serial1.setup(460800, { rx: D1, tx : D2 });
  setupWifi();
}


function setupWifi() {
  var WIFI_NAME = "UseThisOneMom";
  var WIFI_PASS = "asdf";
  wifi = require("ESP8266WiFi_0v25").connect(Serial1, function(err) {
    if (err) throw err;
    console.log("Connecting to WiFi");
    wifi.connect(WIFI_NAME, WIFI_PASS, function(err) {
      if (err) throw err;
      console.log("Connected");
      // Now you can do something, like an HTTP request
      require("http").createServer(onPageRequest).listen(8080);
    });
  });
}

function onPageRequest(req, res) {
  var a = url.parse(req.url, true);
  var f = E.openFile(a.pathname, "r");
  if (f !== undefined) {
    res.writeHead(200, {'Content-Type': 'text/plain'});
    f.pipe(res); // streams the file to the HTTP response
  } else {
    res.writeHead(404, {'Content-Type': 'text/plain'});
    res.end("404: Page "+a.pathname+" not found");
  }
}