How to send a large amount of data faster ?

Hi all,
I am working on a webserver using espruino and the esp8266, to store sensor data in the flash, and serve it to a web browser. But the network seems very slow, e.g. about 1kb per second, depending on the size of each write, much slower than when using Arduino. In this example, it takes 15second to serve 20kb :

function onPageRequest(req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  var n = 4, chk=5*1024, t=getTime();
  res.on('drain',function() {
    if (n==0) res.end();
    else {  
          res.write(E.toString(new Uint8Array(chk)));  
         n--;  }
 
    print("t:",(getTime()-t).toFixed(3),"sec");
  });
 
}
require('http').createServer(onPageRequest).listen(8080);

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 1v95.23 Copyright 2017 G.Williams
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Flash map 4MB:512/512, manuf 0xef chip 0x4016
>
=undefined
t: 0.021 sec
t: 5.057 sec
t: 9.288 sec
t: 13.494 sec
t: 17.717 sec
>

I tried with Arduino, it was 100x faster, and served 1.5Mb in the same 15 seconds. So it cannot be a hardware limitation. Is there a workaround or another way to send data faster with espruino ?