This is related to: https://bugreports.qt-project.org/browse/QTBUG-28782
That bug is about the default not being subpixel hinting on windows.
The subpixel hinting itself (this report) is not really looking good. Even with subpixel hinting the fonts look blurred and you can visually see red/green/blue lines. So i guess the subpixel hinting algorithm needs to be tweaked to get smooth sharp fonts without (noticeable) r/g/b artifacts.
Btw. where in the code is this new subpixel hinting done anyway? I'm guessing freetype isn't being used in scenegraph?