Loading a webapp in different environments

I have developed a webapp that is hosted online. I also have setup a firefox and chrome extension to load this webapp in a sidebar/popup.

When I open the sidebar in the browser it takes about 2 seconds for the webapp to appear in the sidebar.

What would be a good way to show the user that the webapp is loading? Do I have to put he "loading..." script in the webapp itself or in t开发者_如何学JAVAhe browser extension?

Any other suggestions?

Good UI would be to notify the user of what's going on. A simple spinner (http://ajaxload.info) would do the trick. Just show/hide an absolutely positioned div on load :)