Flying Fox wrote:
a delay and the screen goes all dark on you with the dialog, which is intended behaviour to alert you that the application is asking for elevated access. Something that requires your attention.
The delay is more than a delay. It's also intended to secure the input channels and prevent "malicious" permissions, for example via mouse cursor hijacking. Also, while the UAC prompt is in display, Aero is disabled and all windows and screen updates are suspended.
This makes it seem like the computer "jerks" back to work after you make the prompt go away with whichever answer, but UAC itself actually has little effect on performance.
There is, however, the problem with "checking" executables - the larger the .exe, the longer it will take for UAC to finally pop up. If you have a one-gig Windows service pack setup for example, it may take over 30 seconds between the double click and the actual running of the program, and this is directly a performance result of UAC. Without it, all executables are loaded instantly. (This is especially irritating over networks with a slow switch or otherwise modest connections, because even medium-sized setup files take next to forever, if you're installing them from the network. For that, you should copy them over to the end machine first.)