World chunks not rendered

Several problems with chunk generation/rendering:

• Chunk creation is very slow (was much faster in older versions)
• Far away (and deep down) chunks are created before those immediatelly at my feet. (see screenshot)
• I assigned 4 GB of memory to Minecraft, but it only uses around 400 MB. It could keep much more chunks in memory which would avoid re-generation.

This is Windows 7, 64 bit Java 7. 4-core, GTX670. Happens in Survivial and Creative. I am playing single player. In older versions I had a renderdistance of 32 on this computer without any visible problems, so the machine should be fast enough.