I've noticed on my system, the one thing about pubuntu that's slower than ANYTHING else is gksu. Every time the password prompt comes up, I have to wait about 20 seconds for it to fill the screen with black before I can even start typing my password, then another 20 seconds to wait for the black to be removed afterward. Since I figure I'm probably not the only one with this problem, I figured I'd post a helpful tip to fix it. It's pretty simple.
1. Open a terminal and type "gconf-editor"
2. In the window that pops up, expand the "apps" folder on the left, and scroll down to find "gksu." Click on it.
3. In the right hand pane, check the box labeled "disable-grab."
Note that this does introduce a bit of a security flaw - the "grab" is used to prevent malicious applications from listening when you type your password. But disabling it will remove the black background, thus eliminating that terrible and painful delay while it gets drawn.
THX a lot that was the first thing i was looking for :)
it works :)