The Biome Blend feature has been added, and I do love it as it makes the terrain much more attractive at 15x15 blending, however, at 15x15 there is significant lag when generating and loading chunks. It becomes more apparent at 32x32 render distance, but it's bad. I did an experiment. With FRAPS benchmark tool, I recorded the FPS I was getting at both 15x15 blend and no blend. For the purposes of the experiment I turned off V-SYNC and set the FPS limited to unlimited. With my system, I should get 60+ FPS constantly.
Without blend, I got 153 FPS on average,
With 15x15 blend, I got a measly 48 FPS on average, a third of what I was getting before.
This feature needs some optimization.