I've had to stop using nUI lately due to framerate loss in cities and battlegrounds.
At first I thought I just needed a reboot, or to restart WoW. Then I suspected inefficiency in the mod may be the cause of the low framerates. I did a test - I turned off nUI and turned back on ICE HUD, Forte and all the other mods I had used prior to using NUI.
I consistently had 60 fps sitting still in the same location in Dalaran, not moving the camera or anything else. When I spun the camera around it lowered a few frames, but nothing visually noticable.
Then I logged out, turned off the other mods, re-enabled nUI, and logged back in the exact same location, same camera angle, no movement of any kind. 36 fps. I spun the camera around it it was worse.
To validate again, I logged out agian, turned off nUI, re-enabled the other mods and logged back in. Back up to 60 fps.
Conclusion - nUI has some inefficiencies that are causing significant loss of framerates. Trying to do competitive pvp in such an environment just doesn't work, so I had to disable nUI.
Love to see this fixed so I can try it out again.
(I have a great system and run at high detail, maxed clipping, etc.)