I far prefer Qt to Gtk, but even so your arguments are specious.
Your 'benchmark' is two entirely different applications, so the speed
differences could be (and probably are) due to implementation differences
completely unrelated to the choice of toolkit.
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). Again, unrelated to
anything that could be connected with the toolkit implementation language
and only distantly related to the choice of toolkit (Cairo is not part of