The impact of these multi-threading improvements to VP9 are very exciting. Straight from the mailing list message discussing the work, "encoding HD videos with 4 column tiles, the improved VP9 MT encoder achieved speedups over the original of 11% with 2 threads, 27% with 4 threads, 101% with 8 threads, and 135% with 16 threads."
This is due to libvpx's VP9 encoder switching to using a block row based threading approach. This has allowed over a 100% speed improvement for 720p/1080p videos with using more than four threads, faster encoding for small resolution videos, and greater than 10% speed improvement when there is no change in the number of encoding threads. Check out that mailing list post for more details.
Those interested in making use of VP9 within FFmpeg can find Google's recommended settings here.