I think the reason is because they load up so many areas at once. Unlike Mario etc., large portions of the overworld are loaded at the time for interconnectivity between many of the routes and cities without having to go through gates.
Gates exist in the previous games for loading purposes, to reload the next area and all the assets within it. That is how they went about loading.
My bet is, with XY, they tried to do this and keep the stereoscopic 3D, but the processing of rendering all those areas in realtime, twice, caused an unacceptable performance drop.
Every game previously listed either has segmented levels, or like Ocarina of Time, have separation between areas to do this loading.
In the image you linked, it showed Route 2, going into Route 3 which leads into Santalune City. In addition to this, it's almost certain that Santalune City connects to other routes in a similar manner due to no sign of any gates
Also, as the camera is more dynamic, they need to make sure everything is rendered and saved in the position it was at when you last intereacted with it.
It may seem inconsequential, but it adds up.