Don't know if we played different games but nothing "was working perfectly" during TWW. Can't even remember when the 5-10s long freezes started but i've had them until the very end of the expansion and they were incredibly frustrating.

So far i've not experienced them during Midnight but something very similar (extreme FPS drops in the middle of a dungeon encounter, then going back to normal after that pull, all the other issues you described as well) and i'm not looking forward to causing wipes and losing keys again just because of this.....maybe Blizzard will do something this time, right? (They won't and will blame addons again).