Compared to the 19 May libvpx state, Aylesbury is 20~40% faster with its decoding speed and there's a 7% overall PSNR (peak signal-to-noise) improvement with VP8's best quality encoding mode. There's also up to a 60% improvement on very noisy, still or slow moving source videos, according to Google engineers.
Making some of these improvements possible are single-core assembly optimizations, threading improvements for multi-core CPUs, improved memory handling, a smaller memory foot-print, combining IDCT and reconstruction operations, and SSE3 usage where supported.
The next VP8 Codec SDK release from Google is expected in Q1'2011 and is codenamed Bali. Details of the Aylesbury release can be found on the WebM blog.