My general understanding is that Linux has become something of a second-class citizen with Qt4/Qt5; rather than optimising the graphics code for X11 as Qt3 did, it uses the CPU to draw all elements, then only uses the graphics card for certain 3D operations and transformations. While I don't have all the links handy right now, a quick Google search turned up these related links: https://bugreports.qt-project.org/browse/QTBUG-19636?focusedCommentId=177704... http://phoronix.com/forums/showthread.php?73736-Qt5-s-Linux-Requirements-Cau...
This may change in Qt5, but would then rely heavily on powerful, non-free graphics hardware to function (scenegraph backend). As another ancdotal datapoint, I have noticed that Qt4-based applications have always been significantly slower than equivalent Qt3 applications. Note that "equivalent" in this case means displaying a similar number of on-screen widgets; many Qt4 applications have had so much functionality stripped out and/or hidden behind a bulky "touch" interface that direct comparisons are quite difficult.
Yes, it is true. You can even compare it with programs that haven't changed at all since KDE3 like KPPP and Ark. The performance is worse on Qt4.
Between the noted performance problems and the fact that the Qt project seems to be completely unable to resolve several bug reports related to incorrect graphics primitive drawing (for example, https://bugreports.qt-project.org/browse/QTBUG-25896), it would appear that the TDE project's decision to stick with Qt3 was the correct decision.
Tim
Yes, I 100% agree that the decision to stay with Qt3 is the right one! The performance of Qt3 is a big part of the reason why I prefer TDE over KDE. It is amazing to see everything appear just as soon as you click on it!
Thank you for your hard work on TDE! -Alexandre