GTK 2 overall performance
Lets begin our investigation running two different themes, Mist and Clearlooks. The former is a very simple, high speed theme, while the latter is the default Gnome (and Fedora) theme, providing some pleasing elements while remaining quite simple. We also use two extremely different fonts settings: a first, very low quality setting (no antialiasing and no hinting) and a second, high quality setting (subpixel antialiasing and full hinting).
So, lets examine the themes difference first. As you can see, the more pleasing Clearlooks theme is only ~3 seconds slower than the simpler Mist theme. However, while this seems a minor difference, it demonstrate that theme selection have a impact on performance indeed. Using much more complex themes (as some Murrine-based ones or, speaking about Ubuntu, the Ambiance-based themes), I recorded greater speed differences (about 5-10 seconds).
Anyway, the fact that various theme engines can lead to different results is hardly surprising. So, lets concentrate on fonts settings now. From the graph above we can see a strange picture: the most complex fonts settings is marginally slower that the most simple one! While the differences are into the error margin, these results are repeatable and demonstrate that subpixel antialiasing and full hinting aren't a performance bottleneck here.