I love this feature. To access it, type "Performance Information and Tools" in the search box on the start menu and press enter. On the left, under tasks, click "Advanced Tools". Under Performance Issues, if there are problems with a device driver slowing down your computer from waking up from sleep mode or entering sleep mode, it’s listed! Using this, I discovered a program that was causing my computer to take an extra 20 seconds to go to sleep. Vista claimed only 10 seconds, but according to second hand on my wrist watch, it was more like 20 seconds saved. You can bet I turned off that program quickly :). On my desktop, it isn’t nearly as interesting – it just suggests lowering my visual settings because I have a weak graphics card.
Something small, but even many power users would have a difficult time figuring out performance issues like this, and Vista does it for you! Thanks!