I had this issue as well. I was only able to get past it because I was overleveled for the encounter (cleared everything I could get to in Hinterlands before I started any major quests).
Are you guys using the 60fps command line tweak that has been circulating?! I was having a lot of issues (particularly with cutscenes) until I removed this tweak... I wonder if it caused this problem as well. If you have that tweak enabled I highly recommend removing it even if it doesn't resolve your issue with this encounter!! It completely ruins the epic end of ACT 1 cutscene so definitely turn it off before you get to that point.
Np, I never used that tweak. Everything was working fine until this boss fight. The first time I fought it, it worked fine, but I died and had to reload. That's when the issue started. I had the same issue as the OP with the little lag leading up to it when you discover the area.
