Game goes unresponsive for 15 seconds at a time

So the game will simply freeze and go unresponsive during big battle events. It happened on Nathanos as well as any icecrown rares. I have had lag before and expect the lag, but never had my whole game freeze and go unresponsive before. It last for a good 15 seconds or so then comes back. Anyone else having this issue?

Seeing lag every time a rare mob spawns in. First second or two is fine, but as soon as abilities start going off it becomes a slideshow. Confirmed it with multiple people in my group and across multiple servers.