I am currently using a beta version of Xfce4.4 and I find desktop rendering to be a bit sluggish sometimes. It is most visible in times of greater CPU usage, when some program is working on something (like for example pacman installing new software).
If you drag a window, like a terminal in this picture, it tends to leave a trail of white behind it. Sometimes after switching from a heavy maximized Firefox window to an empty desktop it takes a few seconds for the wallpaper to show displaying whiteness in the meantime.
