So since 5.0.4 I've had this problem. The game is fully installed, but sometimes the UI and loading screens will act as if they're being streamed (the icon, lagginess, and everything) and even the world sometimes. It's been progressively getting worse though...
Today I was kicked out of my 3rd LFR for being in the load-in screen for 20-30 minutes.
Just got my Monk to Outland, and it's... very very ridiculous. It's as if I'm having to re-stream the entire Burning Crusade expansion.
Is anyone else having this problem, or are any fixes known?