To find the time spent per question, we need to track the time between firings of showQuestion and registerAndShowNextQuestion.

time_per_question.js

var oldShowQuestion = sm.showQuestion;
var oldRegisterAndShowNextQuestion = sm.registerAndShowNextQuestion;
var addTimingInfo = function(q, tag) {
  var start = document.createElement('input');
  start.type='text';
  start.id=tag+'_'+q.id;
  start.name=tag+'_'+q.id;
  start.form='mturk_form';
  start.hidden=true;
  start.defaultValue=new Date().getTime();
  console.log(start.value);
  document.getElementById('mturk_form').appendChild(start);
};
sm.showQuestion = function (q) {
  addTimingInfo(q,'start');
  oldShowQuestion(q);
};
sm.registerAndShowNextQuestion = function (pid,q,o){
  addTimingInfo(q,'end');
  oldRegisterAndShowNextQuestion(pid,q,o);
};