I have a custom void world (The world was generated and mostly built on MC 1.8) and in most areas I visit, it runs just fine. At least 30 FPS, all normal. However is some areas, I don't know what about these areas, I get 0-1 FPS. I don't know if Minecraft is converting entity formats or something, but I let it run for about 30 minutes to see if it resolved itself, and it didn't. Again, many areas are running perfectly normally and fine. Some areas are not. The areas that are not running fine aren't the ones you'd think, either. They contain much fewer entities than other portions of the map and are overall less complex. This issue is exclusive to 15w31, and was not a problem in 1.8.

Minecraft Console Log:

[10:33:20] [Client thread/INFO]: Setting user: Avantir_Yihn
[10:33:20] [Client thread/INFO]: (Session ID is token:23b5762b01e34694a30149fcab808d2b:99207df7e1b24c75a4cdb05f83c897d4)
[10:33:22] [Client thread/INFO]: LWJGL Version: 2.9.2
[10:33:22] [Client thread/INFO]: Reloading ResourceManager: Default
[10:33:22] [Sound Library Loader/INFO]: Starting up SoundSystem...
[10:33:22] [Thread-9/INFO]: Initializing LWJGL OpenAL
[10:33:22] [Thread-9/INFO]: (The LWJGL binding of OpenAL. For more information, see http://www.lwjgl.org)
[10:33:22] [Thread-9/INFO]: OpenAL initialized.
[10:33:23] [Sound Library Loader/INFO]: Sound engine started
[10:33:24] [Client thread/INFO]: Created: 512x512 textures-atlas
[10:33:30] [Server thread/INFO]: Starting integrated minecraft server version 15w31c
[10:33:30] [Server thread/INFO]: Generating keypair
[10:33:30] [Server thread/INFO]: Preparing start region for level 0
[10:33:31] [Server thread/INFO]: Changing view distance to 20, from 10
[10:33:31] [Server thread/INFO]: Avantir_Yihn[local:E:59af46a5] logged in with entity id 5 at (478.2989286197563, 151.6789996035237, 283.2471044362154)
[10:33:31] [Server thread/INFO]: Avantir_Yihn joined the game
[10:33:33] [Server thread/WARN]: Can't keep up! Did the system time change, or is the server overloaded? Running 2893ms behind, skipping 57 tick(s)