Before the introduction of the native API, we detected font loading by inserting test elements in the page. These elements were regularly polled for width to see if the font had loaded. This process is very expensive because it triggers a page wide style reflow each time the elements are checked. Using the native API, we can bypass this process completely, which results in a big performance improvement.
Always nice when browsers step up and help make things better and faster, especially when there was demonstrably high need, and then see major vendors make use of it right away.
Webfont stuff is a fairly regular topic around here, feel free to browse around.