Although I am running tekkit at the moment, I had this problem for vanilla as well. Everything in the game is sped up; movement, animation, weather, time, mobs and animals. Everything is jerky and fast. At first I though it was just some sort of strange lag that my PC was constantly trying to catch up with but at one point I just stopped moving and looked up at the sky. The starts were moving super fast in a smooth motion. Im not sure whats making this happen but it makes gameplay crappy.
System specs:
CPU - AMD FX 8350 8core 3.7 GHz
GPU- GeForce GTX 660 3GB DDR5 192bit
Windows 7 64bit
Java- ver7 update 21
System specs:
CPU - AMD FX 8350 8core 3.7 GHz
GPU- GeForce GTX 660 3GB DDR5 192bit
Windows 7 64bit
Java- ver7 update 21