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I was doing some tests with different parameters to libvpx. Graphed at this location: http://imgur.com/a/OUiAV are the SSIM by bitrate and SSIM per second of encoding time (a metric I made up, but it's just the SSIM divided by the number of seconds) by bitrate of both vp8 and vp9.
The test clip was the 480p Sintel Trailer uncompressed.
As expected, vp9 beats vp8 in SSIM by quite a bit for each bitrate, but when encoding time is taken into account, vp9 still takes longer than it does make it better.
I don't do this sort of analysis often, so please critique my methods. Especially if you can think of a better way to measure the performance of the encoder taking into account the time to encode.
I got many requests to turn webm into a webm submission subreddit and it makes a lot of sense. I personally still want a place for intelligent discussion about WebM news, techniques, etc, so I think this is a better subreddit for that.