whenever it's happened to me, it's been the first time i've tried to access dalaran from my class hall for the day.
i usually log out in my class hall, then, when i log in, do my class hall champion board thing, then take the portal to dalaran - this is where i run into ridiculously long loading screens.
last week at home, it was only 2 to 3 minutes, today, after 15 minutes (on my work pc, a little slower than my home pc but not by much) i quit and restarted wow only to find myself in dalaran.
additionally, after i've been through this long loading screen once, subsequent loading screens between my class hall and dalaran are as quick as you would normally expect, it is literally just the first time in a session that it happens.