Website loading speed affects the traffic on your site.  How many times have you clicked on a link to a website where the response is slow?  Most often people will click the back button and find a site that will load faster.

A way to make websites faster is to use an asynchronous user interface (AUI).  This just means that the page doesn't reload when the user clicks on a link or a button.  Think of gmail (asynchcronous) vs hotmail (synchronous).  This results in considerably faster interactions due to the user continuing to interact with the webpage while the application works in the background.

It's worth mentioning that AUI is not the answer to everything.  For example, credit card information should be synchronous and blocking.

Hopefully after reading this you won't be surprised when you hear the words asynchronous user interface or single page application.