Edit in JSFiddle

function checkUploadSpeed( iterations, update ) {
    var average = 0,
        index = 0,
        timer = window.setInterval( check, 5000 ); //check every 5 seconds
    check();
    
    function check() {
        var xhr = new XMLHttpRequest(),
            url = '?cache=' + Math.floor( Math.random() * 10000 ), //prevent url cache
            data = getRandomString( 1 ), //1 meg POST size handled by all servers
            startTime,
            speed = 0;
        xhr.onreadystatechange = function ( event ) {
            if( xhr.readyState == 4 ) {
                speed = Math.round( 1024 / ( ( new Date() - startTime ) / 1000 ) );
                average == 0 
                    ? average = speed 
                    : average = Math.round( ( average + speed ) / 2 );
                update( speed, average );
                index++;
                if( index == iterations ) {
                    window.clearInterval( timer );
                };
            };
        };
        xhr.open( 'POST', url, true );
        startTime = new Date();
        xhr.send( data );
    };
    
    function getRandomString( sizeInMb ) {
        var chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789~!@#$%^&*()_+`-=[]\{}|;':,./<>?", //random data prevents gzip effect
            iterations = sizeInMb * 1024 * 1024, //get byte count
            result = '';
        for( var index = 0; index < iterations; index++ ) {
            result += chars.charAt( Math.floor( Math.random() * chars.length ) );
        };     
        return result;
    };
};

checkUploadSpeed( 30, function ( speed, average ) {
    document.getElementById( 'speed' ).textContent = 'speed: ' + speed + 'kbs';
    document.getElementById( 'average' ).textContent = 'average: ' + average + 'kbs';
} );

updates every 5 seconds
<div id="speed">speed: 0kbs</div>
<div id="average">average: 0kbs</div>













































































































































































































<script>
    initializeHeader( 
        'estimate users upload speed without direct permission',
        'http://stackoverflow.com/q/15472251/918414'
    );
</script>