I downloaded the newest version of DDU and ran the Clean and Restart. After the restart, I installed a recent NVIDIA driver (416.94).
I did another restart just to be safe.
Then, I ran WoW. Again, I stared at a black screen for ~20 seconds before the game finally showed me the character select screen.
e.g.
I still had the loading issue where the loading bar quickly fills to 100% and then stays there for ~10 seconds before letting me into the game.
I noticed I could open my tradeskill window with 0 freezing. However, after I relogged on the same toon and tried opening it again, the game froze for a few seconds.
Lastly, I removed my Cache folder and all these problems went away. However, looking at other forum posts, this is only a temporary solution.