How to send a large amount of data faster ?

Hmm, that's interesting, it seems hardware related. Your Sparkfun Thing dev-board rocks with espruino !
I don't get your result, with both my poor nodemcus-e12 v3 with 1.95.0 not 4mb , and my v4 lolin with 1.95.20 4mb.
Where can I find this flash map 256/256 you used ? I'm still getting data as slow as before:

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|   __|_ -| . |  _| | | |   | . |
|_____|___|  _|_| |___|_|_|_|___|
          |_| http://espruino.com
 1v95 Copyright 2017 G.Williams
Espruino is Open Source. Our work is supported
only by sales of official boards and donations:
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Flash map 4MB:512/512, manuf 0xef chip 0x4016
>Start connection process
Try Connecting to WiFi  Gardening, cheaper than therapy
=undefined
Test for error
Test for error
Test for error
Test for error
Test for error
null
connected? err= null info= {
  "ip": "192.168.2.8",
  "netmask": "255.255.255.0",
  "gw": "192.168.2.1",
  "mac": "a0:20:a6:14:52:af"
 }
Wi-Fi Connected
n= 4 t: 0.015 sec
n= 3 t: 1.701 sec
[WS] "Hello to Espruino!"
n= 2 t: 3.207 sec
[WS] "Hello to Espruino!"
n= 1 t: 4.681 sec
[WS] "Hello to Espruino!"
n= 0 t: 6.207 sec
[WS] "Hello to Espruino!"