Borders antialiasing for qt?

[nick87720z]

I recall mentioning of drawing technique, used in qtcurve, in which background was painted with antialiasing, but border is not. I honestly can't understand, how much speedup may be from such degradation, since it is just a contour (disabling it for filling would be more resonable for this case). I just looked carefully to qt4 buttons and noticed result: very tiny edge of antialiased background is visible after unsmoothed border on corners.

With gtk2 all looks fine, as it uses antialiasing. And also for qt4 corders with rounding below max (extra or below) on buttons have pure visual roundness, comparing to gtk2.

p.s. First time i guessed it is pure antialiasing quality in qt4 version, yet when i used max rounding. Looks strange, seens it is known, that qt4 supports even subpixel graphics, while for cairo it fallbacks to usual method.
gtk2:
qt4: