Ubuntu's Unity Decision Affects 2D Performance Too
With Cairo there's at least some good news in that for certain Cairo workloads with the Xlib back-end that the composited Unity desktop leads to slightly faster results than using Unity 2D.
However, with other Cairo workloads the performance was the same or more favorable with the defenestrated Unity 2D.
When running some Qt benchmarks with the OpenGL back-end, Unity was fortunately faster than Unity 2D.