Game client gets stuck on 'building terrain' screen

When I start up Minecraft normally and enter an existing world, the client gets stuck at the building terrain.

What I expected to happen was to get into the game.

What actually happened was that the client simply froze at the 'building terrain' part and then after a while it reverted back to the main screen.

Steps to Reproduce:
1. Open Minecraft.exe
2. login
3. select singleplayer mode
4. enter an existing world