Yes, it's hard to find a real-world comparison for QT and GTK. I just gave one example of a QT program which works fast enough.
"And your microbenchmark compares something that uses OpenGL (thus hardware
acceleration) with something that uses Render (thus completely different
and until very recently unaccelerated pathways)."
That's not the point of my argument. QT can use accelerated OpenGL tesselation (it can draw complex curves and polygons with hardware acceleration), but Cairo can't do it. Architecture of QT right now is simply much better.