Since a transition area is just a connecting area between 2 major zones, there isn't anything stoping the player from flying high up in one zone to the point that they could look into the other zone over the transition zone. That would mean the other zone's assets have to be loaded in already while being in the first zone which isn't possible if they are making full use of the memory for each zone which they of course would be.
The only way to prevent seeing over the transition zone would be to make a very tall transition zone that maybe zig zags so you can't see from one end to the other while flying. Like a canyon pass between two tall mountains or something, but that would need to be similar in every area that you can fly in.
The second thing is dependant on each persons hardware so it could load very fast for you but people on older hardware would take along time and they need to make that zone long enough for someone on their minimum required specs to be able to run through seamlessly. Otherwise the would need to raise the minimum required specs and that could make some people not able to play well anymore. In general any company wants as many people as possible to be able to play their game because that's more possible players and money so they would try to avoid doing that unless the pros of limiting players on older hardware outweigh the cons (like the PS3).
They probably wouldn't need to raise the minimum required specs though, they just need to adjust the length of the transition zone but that would absolutely mean they couldn't increase mount speed later like they recently did. They could do what you mentioned and make them slow down when going through it but I imagine people wouldn't like that and complain about that a lot since not everyone knows how that all works and if they did then it would kinda break the illusion and be obvious to them every time they run through.
There's lots of pros and cons and possible solutions but it's all about if the end result is worth changing something they have working already. Not to mention, I've mostly been talking about the map limits to this, there would definitely be more work needed on the back end system stuff to make this work also, stuff I wouldn't really know about.
Man I type too much, I gotta work on shortening these