Do <iframes> and images loaded via AJAX delay window.onload?

I'm trying to make the window.onload event fire sooner so that Google will think my page loads faster (this is a frustrating task since how long it takes to get to window.onload is basically irrelevant from the user perspective, but I digress)

However, I don't know what delays the onload event! Specifically:

1. If I load a Facebook likebox on my page in an <iframe>, does its loading delay the onload event? What about if the likebox iframe has to load a bunch of profile pics; does onload wait until they fully load?
2. Suppose that on document ready I do an async AJAX request for an HTML blob and inject it into the page. If this HTML blob contains a bunch of <img> tags, does the onload event wait for all of these to load?

In general, how does the browser know when to fire the onload event? What sorts of thing开发者_开发知识库s block onload, and what sorts of things don't?

a) You can't control window.onload except by reducing the page "weight". Its up to the browser to decide when its going to declare that event.

b) Google doesn't have a clue about the window.onload event because its not parsing JavaScript.

1) You can completely eliminate the Facebook payload by using XFBML version of the like button and asynchronous loading of the Facebook JavaScript SDK (http://developers.facebook.com/docs/reference/javascript/FB.init/). Do note that it will work only if JavaScript is enabled.

2) Everything that is going to dramatically increase the weight of your web page should be loaded asynchrouniusly, preferably after the window.onload event has fired.

If you look at the waterfall, in firebug or chrome inspector, iframe and ajax calls does affect the onload event. I ran into similar problem with facebook considerably slowing down site. Yes, while looking at pageload time in webmaster tool, it shows the lag.

My solution was to dynamically append facebook iframe when the page is completely loaded. and for ajax calls, i only trigger them on load.

This brought my page load time from 7 seconds with embedded facebook iframe, to 2.6 seconds with dynamically appending it.