In an interesting article at ComputerWorld, Microsoft discusses their new Metro UI and how efficient it is at doing basic, everyday tasks. In turn, this "20%" number is thrown around a bit, with the claim that it takes much less effort to use their mobile OS over others.
While 20% is numerically hard to substantiate, is easy to validate in certain areas. The appeal for Windows Mobile, for many, was the Today Screen, where you could pack a ton of info to your liking so that with a one-look glance you knew what was up in your world. Having easy access to the calendar when you turn it on, one-button search (something Android actually pioneered), as well as dynamic tiles continues this tradition in WP7. The good news is it sounds like Microsoft took a page from Palm's book, where they notoriously would count the number of "taps" it took to do a task and aim to reduce that as a metric for efficiency.
CW goes to mention that this whole simple UI/20% efficiency thing is the basis for their new ad campaign. Who knows, maybe Microsoft is onto something here. Palm's Garnet/WebOS and the iPhone all do well for their simplicity and elegance, the same rule should apply to Microsoft.