I can tell you with 99% confidence that the black loch specific lag is the water related. When I run that quest I pull my camera down so it is near the water and I am looking up at my character and thus not seeing any water. As long as I keep the water off of my screen I do not lag. If I goof and move the camera so the water is rendered the game slows.
I don't know what is special about that water. I can look at the water everywhere else and not have any issue, but there is something in THAT water...
p.s. I also see the affect you mention during Friends in Low Places, but I do not believe the Black Loch issue is caused by the archers.