I only use a few things in AUI. For example, the flying numbers for damage etc and the buff trackers i use the stock UI elements. Mostly i use AUI for frames so I can move things around. And have different Attribute bars.

I do not notice lag differences with AUI on or off. So, if your using it for everything, try replacing what you can with what the stock UI offers.

I use oblivion-style UI. the only thing i wanted from AUI was the ability to change the enemy bar colors, and there's an addon that just does that, so I got it. It's called targetcolor (so red isn't non-aggro, but the other way around.)