All entities lag in superflat worlds, whether they are default or a custom preset.

Every entity, from Rabbits to Minecarts, jumps from one place to another. It is in no way smooth. I have 30+ fps at all times when this happens and nothing else in the world lags. I have tested in a regular world. I have not tried this in Custom or Amplified worlds, however, it does not effect Large Biomes. It does not seem to effect 14w27a.