I don't think closing this as invalid made sense. I've also seen massive lag with riding horses:
Using JDK 1.6 on the client side seems to be massively worse with riding than JDK 1.7
The network performance requirements go through the roof when riding, where I can ride nicely on my server when at the office, but get noticeable lag when riding at home with a slower Internet connection
Haven't debugged the server settings yet, but FWIW, riding seems to cause Minecraft consume a lot more resources than before.
Happens every time when I'm hosting my own server with snapshot 13w17a. (Also all manually spawned horses despawn 100% of the time when reloading world, intended?)
Tails I did search - how about adding relevant keywords to the master issues? What of the following did you expect to pop up in a search for collision issues:
"if im riding on a pig i can go through things, that hier, than one block"