I investigated the poor performance QT4 showed at QGears2 in the aliased case. (20fps vs. 80fps antialiased)
QT was using XRender in a quite inefficient way for the aliased case, as well as Xorg falling back to software although it could clearly do better. I hope I can fix that, shouldn't be too hard.
I sent them a mail, and after a short discussion a patch was written by trolltech targeted for 4.5, improving the performance of aliased shapes by a factor 2. Once Xorg is fixed performance should double again.
Sometimes its really a big plus to have a company behind an open source project maintaining it, like its the case for OpenJDK or QT. Some other projects I tried to contribute are in a far worse state.