This is not technically a bug, but since that's the only category you have for posting under i have to choose it.

There is a memory leak in LWJGL that causes the game to become slower and slower if you change dimensions a lot, for instance between Overworld and Nether (also all Mystcraft worlds).
The problem should be fixed in newer versions of LWJGL, so when they get the problems with OSX 10.6+ fixed the LWJGL shipped with Minecraft should be updated.