You'll find many more in this great session by Unity engineer Ian Dundore, Squeezing Unity: Tips for raising performance (section on Unity UI starts at 23:38). Developer EA Motive confirmed as much talking to IGN. The problem is that I have DEF_UI installed via the in-game mod menu (because i cant get it to work when I install it manually) and.
I know that both use the HUD_Menu.swf file and that I have to use the autopatcher from HUD Frameworks to fix this. This approach assures maximum compatibility with uGUI, best performance and makes the entire system behave in a predictable manner.
It does that by manipulating native Unity components and taking full advantage of their intended usage.