Is there any long running fantasy series that maintains the pace of its first two or three books?
I can't really think of one off-hand.
I'd want to say Discworld, but since each book is a self-contained story (albeit with often quite strong continuity to earlier books) it doesn't seem fair to compare it to something like ASOIAF, which clearly has an overarching narrative.
How can any series with over a dozen POV characters and hundreds of other characters be expected to move as lightly and freely as it did at the beginning?
Oh, that's simple.
By having several plots moving simultaneously
This is sort of the point of having many POV characters, isn't it? The problem with ADWD is less having so many POV characters and often not simply knowing when to give them a chapter or not. Bran has two or three chapters that advance his story enormously, Tyrion has a dozen and almost all of them are just there to kill time (with the exception of him meeting Aegon Targaryen, I can't help but feel you could cut every single one of his chapters and pick up his story in the next book and the reader would be up to speed with him by the end of his first chapter). Victarion spends his chapters en route to a destination he still doesn't reach - he could have used one chapter, at most. Generally when a POV character doesn't have anything worthwhile to add, best to keep them out of the reader's eye, and so on.