How to send a large amount of data faster ?

I am in the first case: Esp8266 with Espruino flashed into the Esp8266. By marshalling data, I assume you refer to the E.toString. It's not much different without marshalling. The loop finish in only 13ms with a straight string (instead of 26ms for uint8array), but the 5kb of data still make 5 seconds to finish loading in the browser, in at least ten pieces :

var data="";
var chk=1024;
for(var i=0;i<chk;i++) data+="0";

function onPageRequest(req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  var n= 5, t=getTime();
  for(var i=0;i<n;i++) {
    res.write(data,'binary'); 
    print("t:",(getTime()-t).toFixed(3),"sec");
  }
  res.end();
}
require('http').createServer(onPageRequest).listen(8080);

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Flash map 4MB:512/512, manuf 0xef chip 0x4016
>
=undefined
t: 0.002 sec
t: 0.005 sec
t: 0.008 sec
t: 0.010 sec
t: 0.013 sec
t: 0.002 sec
t: 0.005 sec
t: 0.008 sec
t: 0.010 sec
t: 0.013 sec