Note that upstream considers it a bug ( = should not need xcb), but that's the status quo still in 3.4.1. I have a bug open: https://bugzilla.gnome.org/show_bug.cgi?id=674465
Then there's the size. Comparing gtk3 3.4.1 and gtk2 2.24.8:
libgdk.so 595kb - 455kb
libgtk.so 4.3mb - 3.8mb
I don't have benchmarks, I'm afraid. My definition of bloat is about size, a bloated app may still be faster than a lean one.