The first is that the method Yhwach and co. are using (i.e. the beam of light) makes it possible to get there a lot faster; perhaps it's some sort of separated dimension that works like the Precipice World, or it's simply a channel of fast-flowing reishi that accelerates their movement.
The second is that the barriers are actually the reason why it usually takes so long to get in/out of the Royal Dimension; perhaps the barriers slow down movement, or they work like the Precipice world in reverse (i.e. a few minutes within the barriers equals a few hours or days outside of them).
---------- Post added at 11:47 PM ---------- Previous post was at 11:15 PM ----------
Have a look at this page;
After re-reading the chapter, I believe the barriers actually restrict the speed of anyone going through them; Yhwach mentioned how the cloth protects its wearer from the friction of travelling to/from the Royal Dimension, and I bet it's that friction that impedes their movement. In reality the Royal Dimension may be less than an hour away from the Seireitei in terms of getting there via high-speed maneuvers like flash step, but the barriers impede this movement with such effectiveness that it takes days or more to reach it; it would be like trying to run through treacle!!!
Both of these answers are good. So if the cloth helped with the friction, it helped Ichigo get there in half a day, around 12ish hours. So if the beam of light technique discussed here is used, they must be traveling at hyper sonic speeds. MAKES PERFECT SENSE. Yhwach is a man of total mystery. Ichigos definitely losing an arm. -__-
I think, Sternritter D, Nak Le Varr, has dimensional abilities. He can literally alter the space around him and create passages. It could be similar to Garganta and I imagine, his power will be used to transport Yhwach, Uryuu and Haschwalth to the Royal Palace, now that the barriers are broken.
Also, I bet, Nak Le Varr is responsible for making Sereitei disappear and replacing it with the Vandenreich citadel. Mayuri needs to kill that guy and even this war back towards the Shinigami. I bet, because of this change in location is the reason why, the Sternritter are so powerful in the first place.
Naa, we were all wrong and Bach used the shine star transport system.
My guess is that the difference is the mode of transportation. Ichigo literally walked out of the spirit palace with the aid of his very own spirit key. However getting in and out of the palace has presumably been done by others before. The first time ichigo got there they used the pillar thing and it seemed pretty automatic as far as we saw. It does not seem like a lot of time went on between the end of the first invasion and the time the royals arrived so they probably got there well faster than what it took ichigo. So there is a difference between running out of there and using proper transportation.
my money is on a proper transport as well. i think thats the reason they were sent down without one: RG didnt want to let ywach get his hands on one.
which does raise a question: why do the quincy have a transport up? and even more weird: why is it all quincy style? i highly doubt SS making quincy style transports