Low Performance on Pre-1.6.2 Worlds

I recently updated to Minecraft 1.6.2, and I've notice a major performance loss on worlds generated before 1.6.2. Before I installed 1.6.2, I would get over 120 FPS, the game would rarely crash, and the game would only use about 200MB of ram. With 1.6.2 installed, Minecraft crashes and freezes constantly under normal use. The game is also running off of about 700MB of ram. My FPS rate is very unstable, sometimes it is at pre-1.6.2 rates but it quickly drops to less than 20 FPS. When loading a world, under the Development Console it says "[Severe] GL Error", and when Minecraft crashes it says "Invalid Access of Memory". The Development Console also shows "[Warning] Can't keep up! Did the system time change, or is the server overloaded?".