I was flying around in the Nether, trying to find a place to place down another portal to connect somewhere else in the Overworld, and I was just lagging as if there were tons of entities around, but I was in Peaceful difficulty. When I got to the Overworld in my new portal, the lag was gone.

How to reproduce:
1. Set difficulty to Peaceful.
2. Make sure you are in Creative.
3. Enter the Nether.
4. Start flying around.

This did not happen to me in any previous versions, even on my old slow computer that I stopped using in 1.2.

EDIT: THIS ONLY HAPPENS ON PRE-RELEASE WINDOWS!