worker-latency.js

var bb = new Blob([`
  onmessage = function(e) {
    postMessage('pong');
  };
  postMessage('started');
`]);

var worker = new Worker(window.URL.createObjectURL(bb));
var startTime = -1;

worker.addEventListener('message', function(e) {
  switch (e.data) {
    case 'started':
      startTime = Date.now();
      worker.postMessage('ping');
      break;
    case 'pong':
      var t = Date.now() - startTime;
      console.log('latency is ' + t);
      break;
  }
});

latency is 0