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To those saying GTK+ is slow, you probably haven't tried Xfce. Xfce is lightning fast (much faster than both Gnome and KDE), and it's fully based on GTK+.
Any slowness in Gnome comes not from GTK+, but from all the other libraries, metacity, pango, xlm based config files, and whatever else.
But alas, if you use a distro that is Gnome centric, or treats Gnome as a first class citizen, then Gnome is not slow. I'm currently typing this in FireFox, launched from Gnome 2.8, on a Debian Sarge installation. This installation has both Gnome 2.8 and KDE 3.3. This is on 300MHz, 256meg RAM machine, and both Gnome and KDE are very snappy, with Gnome actually being slightly faster. Now, I've also had Gnome installed with a Mandrake 10 installation, which defaults to KDE. In this case, KDE was faster, probably because Mandrake applied some optimizations to KDE.
So, GTK+ is very fast.
Gnome is fast too, if it's implemented by a distro that makes it a priority.