WebApp Lag Meter non SSI using Ajax

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<script>
function createRequestObject() {
  var ro;
  var browser = navigator.appName;
  if (browser == "Microsoft Internet Explorer") {
    ro = new ActiveXObject("Microsoft.XMLHTTP");
  } else {
    ro = new XMLHttpRequest();
  }
  return ro;
}
var xmlHttp = createRequestObject();
function runping() {
        xmlHttp.open('get', '/?ajaxping&ms=' + new Date().getTime());
        xmlHttp.onreadystatechange = readping;
        xmlHttp.send(null);
}
function readping() {
        if (xmlHttp.readyState == 4) {
                var a = new Date().getTime();
                var ping = a - b - 10;
                if (ping > 350) {
                var pc = 'red';
                var pcode = 'Ultra High lag';
                }
                if (ping <= 350) {
                var pc = 'orange';
                var pcode = 'High lag';
                }
                if (ping <= 225) {
                var pc = 'yellow';
                var pcode = 'Low lag';
                }
                if (ping <= 110) {
                var pc = 'green';
                var pcode = 'No lag';
                }
                if (document.getElementById("ping")) {
                        document.getElementById("ping").innerHTML = ping + 'ms<br /><font color="' + pc + '">' + pcode + '</font>';
                }
        }
        //setTimeout("var b = new Date().getTime(); runping();", 5000); // Has a Error in it for testing only
}
var b = new Date().getTime();
runping();
</script>

<div id="ping"></div>