It's really east to foul up memory allocation and processor use in AE. Rendering over a network can also cause problems. Some codecs don't seem to like 64bit either, others love it. I once allocated 100% of everything to AE then tried to render to a network drive. AE ground to a halt because there was nothing left to push the data over the network.
A complete description of your system and your AE prefs and your rendering setup would help. If all else fails, just turn off multi frame rendering and reduce your memory allocation so that there's at least a GB for other apps.
As Rick said, without more info on the system, footage, effects, settings etc., there is no way to even guess what might be wrong. In addition to CoDecs and the changes in MP behavior, you might see bad performance from a single feature or effect that has only been half-heartedly converted to 64bit or you may likewise see hardware acceleration issues in effects due to how Premiere has been reworked to make use of your graphics card...