Confirmed with several types of blocks.
This is a significant issue on my LAN server. The first person can usually log in, and the world will render, but takes several seconds the second person will usually have a long lag (up to a few minutes) then the world will render, but any more, and it will sit there with a blank sky, and/or kick the person off because "flying is not allowed"
Sorry I didn't specify. No, this is in normal Survival mode.