Ran across an article on MSDN (I know, right?) about the problems with synchronous XMLHttpRequests, and how it actually causes 8.4% of all hangs in IE9. Synchronous is easier to work with, as it flows just like a user would expect. Asynchronous is slightly tougher to use, as it requires handling the response out of [...]
XMLHttpRequest: Learn why Asynchronous is Best is a post from: Sean Walther's Blog