How to send a large amount of data faster ?

@ClearMemory041063, I am surprised it goes a bit faster on your setup. I still got 17 sec for the 20kb, when I tried the two programs you posted. I tried with/out AP/station, no change. What version of espruino are you running ?
The only change I did was 5*4kb chunk, instead of 4*5kb chunks, as the latter stopped after n=3 (I guess a memory shortage).

var n = 5, chk=4*1024;
var tdata=new Uint8Array(chk);
var i;
for(i=0;i<1024;i++){
  tdata[i]=0x30;
  tdata[i+1024]=0x31;
  tdata[i+1024*2]=0x32;
  tdata[i+1024*3]=0x33;
//  tdata[i+1024*4]=0x34;
}//next i
function onPageRequest(req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  var t=getTime();
  n=5;
  res.on('drain',function() {
      print("n=",n,"t:",(getTime()-t).toFixed(3),"sec");
      if (n===0){ 
        res.end();
        print("n=",n,"t:",(getTime()-t).toFixed(3),"sec");
      }else {  
        res.write(E.toString(tdata));  
        n--;
   }//end else
  });//end opr
 }//end function
require('http').createServer(onPageRequest).listen(80);

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| http://espruino.com
 1v95.23 Copyright 2017 G.Williams
Espruino is Open Source. Our work is supported
only by sales of official boards and donations:
http://espruino.com/Donate
Flash map 4MB:512/512, manuf 0xef chip 0x4016
>
=undefined
n= 5 t: 0.004 sec
n= 4 t: 4.339 sec
n= 3 t: 7.640 sec
n= 2 t: 10.941 sec
n= 1 t: 14.263 sec
n= 0 t: 17.558 sec
n= 0 t: 17.559 sec

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

require('http').createServer(onPageRequest).listen(80);

 _____                 _
|   __|___ ___ ___ _ _|_|___ ___
|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| http://espruino.com
 1v95.23 Copyright 2017 G.Williams
Espruino is Open Source. Our work is supported
only by sales of official boards and donations:
http://espruino.com/Donate
Flash map 4MB:512/512, manuf 0xef chip 0x4016
>
=undefined
n= 5 t: 0.003 sec
n= 4 t: 3.330 sec
n= 3 t: 6.626 sec
n= 2 t: 9.945 sec
n= 1 t: 13.246 sec
n= 0 t: 16.599 sec
n= 0 t: 16.601 sec