The clay pan seems to cause a FPS drop by about 10-20. I'm not really sure what the conditions are, in a fresh creative test world the drop seems quite low, only about 5-10. But in an established world it can be as high as 20.

http://imgur.com/a/aqQmW

I have not been able to confirm this phenomenon under their own development environment and play environment.
However, since has received several reports about similar problems, I'm aware that there is "something" problem.
I think, it will take some time for the removal of the cause.

At version beta3, this problem was solved maybe.