I've just tried GNOME 2.14 (and I'm still in it) and I must say that if my eyes aren't fooling me this does seem like a performance improvement. It feels as snappy as my minimal openbox setup actually.
I remember klepas mentioning that GNOME guys are constantly optimizing GNOME to run faster and feel less bloated and he seems to be right from what I see. I think GNOME 2.14 not only runs a bit faster, but starts faster than KDE 3.5. It's really good.
I'm using htop to see how much memory is spent at certain point and it showed only 24 MBs with full GNOME running with one nautilus window (with its own desktop) and one terminal (xfce terminal). I suppose that isn't bad.. I'll have to measure KDE to see. After I launched Xchat that jumped to 130 and after launching firefox it went to 154. Might not be the most accurate, but here I am with one firefox tab, an xchat with 9 channels and two networks open, terminal and nautilus and it's still not reaching 233MB real memory limit.
Have you tried new GNOME? What do you think?