The last few months, when I go through portals, I drop through buildings and ships.
Today I just went through the portal on a level 90 for the opening to Draenor, and immediately fell through the floor and was in the room below.
This also happens to me on the ship in Siege of Orgrimmar on the way to Galakras, I portal through and drop to the base of the boat.
When this happens I have no lag (45 world and home) and 100 fps.