Here is a brief comparison of Xfce desktop and KDE desktop that comes with opensuse linux 12.1. The screen shots for the konsole using top command for memory and system monitor shows the difference. Notable is that KDE Dolphin uses 25MB whereas, Thunar the default file browser for Xfce used 17MB. Half of Dolphin. On CPU and memory Xfce desktop has half resource usage compared to KDE4. I have not used KDE plasma graphics too much on my desktop. KDE4 desktop is configured to use clean looks and use suse look and feel. Having Xfce on an older hardware can make it faster or comparable to a new hardware running latest operating system.
KDE 6, 3.9% 0.30 GB / 2.0 GB
Xfce 2, 3.9% 0.16GB / 2.0 GB
Xfce feels responsive and faster. In fact too fast. The one disadvantage of Xfce is that, sometimes the widgets/panel apps don't work. For example, the window manager had to be started manually at command line. Or the windows did not have close/minimize/maximize buttons. Again, Lock Screen does not seem to invoke any response from Xfce and also resuming after hibernating does not prompt for password by default.
KDE Top and System Monitor
Xfce Top and System Monitor