Several Ubuntu Xfce users have reported a delay when root graphical applications (ie launched with gksudo or gksu) ask for the password. The windows hang for several seconds, up to 40. I’ve noticed that even the clock stops ticking.
This bug is probably related to Xfce compositing manager, in particular the shadows under the popups.
The current workaround is to disable “Display shadows under popup windows” or launch the application with
man gksudo --disable-grab, -g Disable the "locking" of the keyboard, mouse, and focus done by the program when asking for password.
From the UF thread (see below), run
gksu-properties and disable
capture-mode. I’ve looked around, but does anyone have more infos about this workaround?