Exception ticking world: ssp unmodified: slow machine?

What I expected to happen was:
New chunks should load as previous to 1.4.6, and I should be able to fly over them in creative mode.

What happened:
New chunks were not created, causing the game to crash as I flew.

Steps to reproduce:
Graphics FAST
Render NORMAL
Smooth lighting OFF
Anaglyph OFF
Performance POWER SAVER
View bobbing OFF
Advanced OpenGL ON
Clouds OFF
Particles MINIMAL
VSync OFF

Large biomes, structures on, cheats on

Stay within 10000m from spawn
Fly over DESERT

The world where I noted this to occur was created after version 1.3.

Additional:
Due to a moderately slow machine, new chunks sometimes take a long time to load. Since 1.4.6, if this happens while flying, especially over desert terrain, MineCraft may crash entirely if a chunk does not load before entering. I have not ventured more than 10000m from spawn.

I have not had this issue with previous updates, and have had to significantly downgrade the quality of render to make MineCraft playable on 1.4.6 update for either survival or creative. I have included 2 relevant crash reports.

Is this a bug, or a glitch due to a slow machine?