Web Worker Network Connection - https://github.com/collective/experimental.bwtools

check.js

(function(jQuery) {
  jQuery(window).load(function() {
    var cookie_name = '_bw';

    function get_cookie() {
      var keyValue = document.cookie.match('(^|;) ?' + cookie_name + '=([^;]*)(;|$)');
      return keyValue ? keyValue[2] : null;
    }

    function set_cookie(value) {
      var expires = new Date();
      expires.setTime(expires.getTime() + (5 * 60 * 1000)); // 5min
      document.cookie = cookie_name + '=' + value + ';path=/;expires=' + expires.toUTCString();
    }

    /**
     * If we don't have the cookie call a worker to do some measurements
     * and store the results in the cookie
     */
    if (!get_cookie()) {
      var base_url = PORTAL_URL + '/++plone++experimental.bwtools';
      if (typeof(Worker) === 'undefined') {
        return; // unsupported
      }
      w = new Worker(base_url + "/scripts/worker.js");
      w.postMessage({
        base_url: base_url
      });
      w.onmessage = function(event) {
        if (event.data) {
          set_cookie(event.data);
        }
      };
    }
  });
}(jQuery));

worker.js

/**
 * This function performs a synchronous request
 * and returns an object contain informations about the download
 * time and size
 */
function measure(filename) {
  var xhr = new XMLHttpRequest();
  var measure = {};
  xhr.open("GET", filename + '?' + (new Date()).getTime(), false);
  measure.start = (new Date()).getTime();
  xhr.send(null);
  measure.end = (new Date()).getTime();
  measure.len = parseInt(xhr.getResponseHeader('Content-Length') || xhr.responseText.length || 0);
  measure.delta = measure.end - measure.start;
  return measure;
}

/**
 * Requires that we pass a base url to the worker
 * The worker will measure the download time needed to get
 * a ~0KB and a 100KB.
 * It will return a string that serializes this informations as
 * pipe separated values
 */
onmessage = function(e) {
  measure0 = measure(e.data.base_url + '/test/0.bz2');
  measure100 = measure(e.data.base_url + '/test/100K.bz2');
  postMessage(
    measure0.delta + '|' +
    measure0.len + '|' +
    measure100.delta + '|' +
    measure100.len
  );
};