Ubuntu comes with various flavors beside the one that comes with Unity desktop. Kubuntu comes with KDE desktop, while Xubuntu uses Xcfe, for instance. Since there's no apparent difference between Unity & Gnome Shell, would KDE or Xfce offer better performance?
Here's the Unigine suite of OpenGL benchmarks to compare Unity, KDE4 & Xfce performance. The results may be a little surprising...
There's hardly any difference between the 3 desktops. It just looks bad for KDE, but consider the hard numbers. It's just a fraction of 1fps slower than Unity and Xfce. Not something to write home about.
Even for Sanctuary performance is very close when you look at the numbers. A sub-fps speed advantage, if it can be called that, is hardly going to be noticed in day-to-day usage.
It's only in Tropics that we see a more significant gap between KDE and our two GTK+-based desktops but even then it's not much in the scheme of things.
Subjectively, KDE and Xcfe gives a smoother desktop experience. There didn't seem to be any of the pauses that crop up every now and then when running Unity. That said, I haven't tried either desktops for very long so things may turn out differently on prolonged use.