"Can't keep up!" error while loading worlds

I created a new world in 15w41b and tried to open it. It loaded slowly and then crashed, even though it was loading. After it crashed, I tried to re-load the same world another two times. The log for the third attempt is attached.

I haven't tried adjusting the render distance (20 chunks) or lighting (maximum smooth), but I did turn the other graphics settings down to the minimum - no clouds, fast graphics, no particles, etc.

Steps to Reproduce:
1. Create a new world in 15w41b.
2. Load the world.