Since about a week or two, pretty much since scalebreaker launched. My game runs completely smooth until I do certain boss fights. Ones that stand out are Vet Z'maja when the the mobs come out of the portal, and in the final boss of Vet Ruins of Mazzatun when the boss gets around 40%ish health and spawns a big set of adds. The common theme is adds during a boss fight and without fail my fps drops from 100, to about 1fps, then goes to about one frame every 10 seconds and locks up the game. I am still able to cast all my abilities and type perfect messages in chat. I used to run a ton of addons but have since stopped running them, it didn't fix the problem. I have tried everything including uninstalling the game, deleting my usersettiings and addons folder.
The game isn't capping out my resources, my CPU/RAM isn't really going much over 50% usage.
I can't confirm 100% that it's a character issue, but I think it is. I did a few Vet Ruins of Mazzatuns on my StamNecro and ZERO issues, swapped to my DK tank, my main, without fail the lag is back.