Prompt user with their Slow Speed on top like Twitter

Question

I want to display message on top, whenever user opens my page, when it take time to open my site with their slow speed internet connection.

Like Twitter does, whenever it takes time to open page, it display message on top. I want to have some sorts of similar functionality.

What i have tried :-

<script type="text/javascript">
var imageAddr = "imageCheck.jpg" + "?n=" + Math.random();
var startTime, endTime;
var downloadSize = "170000"; // my image size is approx 170KB
var download = new Image();
download.onload = function() {
    endTime = (new Date()).getTime();
    if(endTime > 2) { // 2 seconds
           //displaying message
        document.getElementById("late").innerHTML = "Its taking time to open Page, <a href='http://www.exammple.com' target='_blank'>Check our Quick here</a>";

            //displaying message box
        document.getElementById("late").style.display = "block";
    }
    startTime = (new Date()).getTime();
    download.src = ImageAddr;
}
</script>

I am trying to use this technique here.. Calculate speed using javascript

Alert box pops up when complete page has been loaded, but i want to display it in middle whenever time reach more than 2 seconds, and not to wait for complete loading of page.

Is this possible ?

Thanks