GNOME developer Benjamin Otte has published quite a few benchmarks on his blog showing off the Cairo performance with the Xlib and OpenGL back-ends on Intel hardware and also NVIDIA hardware with the open-source Nouveau driver.
He isn't done with his testing yet and there are no ATI test results yet of Cairo, but if you're at all interested in open-source graphics or care about the Cairo rendering performance with different back-ends,
the results are worth checking out.
Hopefully this will result in greater optimizations within the graphics drivers and Cairo.