Same symptoms, different problem...
I found this post in my search for answers and I'm hoping that y'all might be able to help me out.
I'm having exactly the same problem, but with two critical differences. I have a very powerful machine, and UAC hasn't always done this.
Because I'm running a powerful machine, I have a difficult time believing that it would be forcing software rendering (unless it's forcing wrongly, for some reason; is there a way to check?), not to mention the fact that I feel like software rendering would still be pretty fast.
Secondly, the UAC prompt used to be really fast (one of the things I have really loved about Win7, esp over Vista). I built this computer recently, and the first few days after I loaded Win7 on it, UAC was as fast as it was on my old machine (which is still fast, BTW). After a few days (maybe a week), UAC suddenly started taking 5-7 seconds to dim the screen and bring up the dialog, and the another 5-7 seconds to return after I clicked "yes."
During those few days, I was installing software that I've been using for a long time, none of which has caused this kind of problem before. None of it is weird or low-level stuff. Games, office programs, Blender (a 3D package), anti-virus, etc.
Hope y'all can help.
PS Please move this to another thread if appropriate.