UpdatePanels are a pretty neat part of Microsoft's Ajax Extensions. It's interesting to note how much faster pages *seem* when you remove all visual aspects of the postback. Sure, the postback is still happening, but without the need to re-render the entire page things seem so much snappier.
However to make sure that your page is running as fast as it possibly can, remember to set the UpdatePanel's UpdateMode to Conditional (which isn't the default).
It might seem like an obvious thing, but when you're working on complex pages that contain multiple UpdatePanels, multiple UpdateProgress controls, as well as UpdatePanelAnimation extenders it can be pretty easy to overlook.