Qt & KDE4 speed advantages
Via KDE Developers:
We are apparently assured that KDE 4 is going to be "about speed" (and not bloat, unlike a certain competitor and unlike the impression you'd get from all these new technologies). Daniel Molkentin's blog entry (in link above) also states that the KDE3 codebase that has so far been ported to Qt 4 has run faster. There is also a link to a Qt Development Blog entry about better performance in QSA 1.2.0 over 1.1.3, which use Qt 4 and Qt 3 respectively.