Block lag and FPS loss of around 100fps or more in Amplified survival single-player worlds. The same worlds work great in 1.7.2 and show no block lag and much higher fps.

I just started testing this theory recently, so I only included the 14w*** series snapshots and the 1.7.4 release as having the bug.

*What I expected to happen was...: equal or better game performance as 1.7.2

*What actually happened was...: block lag causing world to be unplayable, FPS drop/inconsistency causing world to feel unnatural

Steps to Reproduce:
1. open 14w04b in singleplayer mode
2. create new Amplified survival mode world with cheats enabled
3. load your Amplified world.
4. hit F3 and do not move.
5. wait a few minutes for the majority of chunks to load.
5. note your avg FPS and begin to break any blocks, tree, leaves, dirt, etc
6.watch for block lag (block will reappear shortly after you break it, instead of disappearing immediately.)
7. save and quit to title
8. quit game and open the minecraft launcher
9. change to a previous 14w*** snapshot version
10. load the same single player Amplified world you created in step 1 and repeat steps 3-9, noting average FPS and block lag
11.change to version 1.7.2 in the launcher and load the same world
10. note fps increase and block lag is gone