"We can never be sure if bending space around time is the right approach until we get feedback from you on the 14w29b snapshot that bends time around space instead. We do that by using the newly implemented threaded render batch system. That’s a lot a fancy sounding words, so if you don’t know what that means, it should improve performance even more. Or crash horribly. But without your feedback, we’ll never know which one is true." (mojang.com)

It crashes horribly.
What keeps happening is that in 14w29b, a world I upgraded from 14w26b to 29b kept having invisible chunks, then I try to load them (F3+A) and the game just crashes :/. Note: I did upgrade it to 27b, 28b, 29a and then 29b (after it started to keep crashing).