I've been thinking this for a while too. I usually dismiss the time travel inconsistencies since I realize they have to speed things up. But this makes no sense. No matter how slow the dead are moving, they would surely be at the wall by now.
The AoD attacked Hardhome in Season 5 episode 8. To put this time frame into perspective, this was the same episode that Cersei was still a captive and had yet to do her walk of shame.
Look at the map below.
View attachment 24165
Hardome is the top circle in the upper right. The distance between Hardhome and Eastwatch is about 1/4 to 1/5 of the distance between Eastwatch and Winterfell. Somehow the dead haven't made it that far in almost two seasons, yet Euron can travel around the world and back in a single episode.
I have been wondering what is taking them so long for a while now.