Hello Otto,
the Battery Power Usage is interesting for people with notebooks and other mobile stuff.
Under this point it makes rarely a difference between using kde, gnome, xfce or lxde.
And the Memory Usage is an important point for all people with machines, which have less than 2 GB Ram.
But also on the old Thinkpad R51 with 768 MB Ram it is possible to work with KDE4. You've to deactivate all eyecandy stuff and then you'll get an admissible performance to use this old notebook for internet and office.
Kind regards,
Holger