@hendrikL, yes Xfce has one of the most effektive Windowmanagers:
Even using shadows and other composite features it is far less cpu-time comsuming than kwin. But you might miss some of the extra features kwin delivers ...
openbox triggers xorgs X server much too often, they should adapt its code base to a more recent version of xorg! This causes xfce4 to be more efficient for old computers than even razorqt.