Right now the worst loading time for PS4 players is Limsa, as there is always a crowd right there by the aetherite.
The engine already has a cap on the amount of characters it renders so loading time from players in the same zone will never be worse than that, as for scenery if you stand in the plaza you can already see most of the other zone up there, the only differece would be the interior of the Drowning Wench which has no decorations (can be seen by going OOB) and while having to load a full version of the upper zone would be more taxing I don't believe it will be by much, the current zones where designed to run on a PS3 with 512 mb of ram, how much will it take to render the upper zone fully? only the devs will know but can't be more that other 512 mb.