This is sort of a side step and just a theory, so I apologize if this doesn't help, but in the last episode (7x06) there are a lot people upset about the time-factor regarding Gendry sending a raven to Dragonstone, how long it appeared Jon, Jorah, Sandor, Thoros, Beric, Tormund, etc stood on the mound vs how long it would take for Dany to get a raven and make it to where they were...
Also, I know the EPs acknowledged the pacing in regards to this and other time-jumps this season, but these particular scenes made me wonder if it was actually mythos related??
Blockquote “We were aware that timing was getting a little hazy,” episode director Alan Taylor told Variety."..."So I think we were straining plausibility a little bit, but I hope the story’s momentum carries over some of that stuff."

Ok, so we know that The Night King was once a human and was turned by the Children of the Forest and allegedly some of the other White Walkers were also human and turned after a betrayal. From there we know they can make "dead" wights (and each can control those they made suggesting kinetic link), animal wights, and human babies that can be made into more White Walkers (how fast do they age??)...

So we know that Bran, now also the Three-Eyed Raven, is a very powerful greenseer and that most of the children of the forest were also greenseers and greenseers are basically like "time travelers" and "astral projectors". Then Hodor's paradox let's us understand that not only this is somewhat predetermined universe in cycle cosmology, but that any two points in time can be inextricably linked (cause and effect from one event to another). And lastly we know that the mark on Bran's hand allows for the Knight's King to become aware when Bran is in his presence and can establish a kinetic link and know where Bran "really" is.
So I had wondered if there is a radius around the Night King that "slows down time"? ---If that would be true, then it would give new meaning to " A Long Night" ---Maybe this also ties into why it takes them so long to move across the "real" North, because time is slower for them? But at any rate the mythology that is featured in the northern part of the story tends to play with space-time, so it doesn't seem irrational to me that this could be another way to do that.
The bigger picture with this being a cycle cosmological story is that the White Walkers are a metaphor for people having to deal with "the sins of the father" --stemming all the way back to the betrayal of the First Men and the Children of the Forest--essentially a battle between man, his family, and nature (magic) itself and so we're witnessing a cycle breaking into a new one. And picking up many of the dead (especially the wildlings via social exclusion) and wanting a dead army plays into the concept of everyone dealing with many many generations of wrong-doing (including the monarchy of the Iron Throne)...
On a more micro level, it's hard to understand what is really behind the motivations of Night King--if there is something more personal there??
The books get more into the reincarnation aspect of everything, than the TV series, but I really like the theory that Cersei could end up becoming the Night Queen--and that perhaps the Night King recognizes her as an incarnate of his former love interest, because it would reconfirm the human aspect of the White Walkers origin, put a more definite human face on them (another love story), and it would also better explain Cersei, as like many characters, by not being in control of themselves--not entirely human...