Lag in resizing top bar

Hi,

I am resizing top bar to max size on load, however the top bar opens in small size and then goes to large size. Not sure why there's a lag of few milliseconds (good enough to be noticeable). Am I missing something. Can someone help here?  

Here's my code:

window.onload = function () {
    var client = ZAFClient.init();
    client.on('app.registered', function() {
       client.invoke('resize', { width:'640px', height: '490px'  });
    });
}