minecarts look like they have framerate drops

Minecarts look like you have a framerate of 10fps but when you move the mouse (or press f3) you still have a lot of fps.

How to reproduce:
place a minecart on rails, make it move.

Video: http://youtu.be/KeqkyyQc1JY
wow, I scrubbed through it frame by frame but I didn't see it, so you just have to trust me on this one.