Seeking benchmark function snippet for tight loop timing

Generally manipulating large arrays of values directly in Espruino isn't going to be very quick, although the notes in http://www.espruino.com/Performance might help you out a bit.

Since Espruino is single-threaded, it doesn't really matter what you do to use up the time if you want a function that takes a while. Simply doing:

function wait1sec() {
  var  t = getTime()+1;
  while (getTime()<t);
}

Would work fine to give you a known delay - however while it's fine for debugging, doing it in any real code is not recommended (See FAQ: Is there a delay function)