So far, they managed a playback speed improvement of 75% (in fps) in Linux 64, and for a 64 bit Mac box, they produced over a 50% speed up.
It's not all plain sailing though:
The initial challenge, primarily pioneered by David and Ronald, was constructing the core decoder and making it bit-exact to libvpx. This was rather challenging, especially given the lack of a real spec. Many parts of the spec were outright misleading and contradicted libvpx itself. It didn’t help that the suite of official conformance tests didn’t even cover all the features used by the official encoder! We’ve already started adding our own conformance tests to deal with this. But I’ve complained enough in past posts about the lack of a spec; let’s get onto the gritty details.
Diary Of An x264 Developer: Announcing the world’s fastest VP8 decoder: ffvp8
NotesOnVideo: Some things you might want to know about WebM and VP8