When a dispenser attached to a redstone clock shoots items in an nether portal, it creates a huge lag. For more information see http://www.youtube.com/watch?v=cSFhVL_ES3M#t=0m16s
MC-1132
Netherportals create a huge lag, especially when serveral entities have to be teleported
Resolved
Fixed
1
Unconfirmed
item, multiplayer, nether_portal, redstone
Minecraft 1.4.2
Minecraft 1.4.3
Retrieved
This issue can also be seen in one of Ethos LP Epsiodes: http://www.youtube.com/watch?v=fNCeUvilpfg#t=9m10s (He plays in 12w37a, but it is still the same issue.)
The problem is that the game searches for a portal in a huge area in the other dimension every time an entity tries to teleport.
@Simon Althaus Thanks for posting all these bugs. I was about to do that today ;D