That's the thing, I didn't see any difference (granted, that was on my quite recent laptop, but that's what I had for testing). Memory constraint was real in my case: I was going to use this on a computer that had either 256 MB or less.
I guess as a rule of thumb if you don't care about memory usage, you don't need to care about 2D graphics performance either.
I don't think that's true - I have a machine with 4 GB RAM & a 2.x Core 2 Quad. I have no memory problems, but I do have lag when using KDE or GNOME that's not present with Openbox. I haven't tried Xfce so I can't comment on it, but I'm guessing it's at least a little better than KDE/GNOME.
I guess as a rule of thumb if you don't care about memory usage, you don't need to care about 2D graphics performance either.