I clean installed Minecraft and tried to make a world. It starts saying "Building terrain" for about 30 seconds, then gets stuck on "Downloading terrain". The RAM usage goes to about 1300 MB during this process. I then finally get into the freshly generated world, but everything is all blue. I look around with a nice 200FPS, but I'm just falling into the void.
Before this, Minecraft would generate the world at a considerably fast speed, but hang when generating chunks very badly.
I have it running on my new SSD with an i7, R9 290 and 24GB RAM, so I don't think it's my hardware... I tried with Optifine, and without, both cause the same result.
This is all singleplayer.