So let's start with: how do you test that Qt 4 is slower than Qt 3? What's your test, where can I get the source to verify it?
The same old diskussion, and it will not lead to a solution. As neither KDE nor TDE or any other DE offers a test suit the user is up to his/her personal impression. Some months ago (was it last year? time flyes ...) you suggested tde to switch to kwin4 and drop kwin3.5. Well, I tried it and posted my results. It's like walking in tar. Just install TDE, change the WM to kwin4 and start working. You could do as I did, set up 2 identical maschines next to each other (I'd suggest you use Intel or Nvidia gc - X61 in my case) and make a one by one comparsion.
Just as a small reminder: do you really think that can be the case? Qt 4 got trimmed to run on smartphones - I have a few of those (e.g. the N9 and the N950, also used to have a Symbian device) and they would love to have the specs of your X61. Given that Nokia invested lots of money in that area, do you really think the engineers would not have realized that Qt 3 was faster, look at that code and make better use of it in Qt 4?
And Nokia is still the rising star of smartphone universe ;-)
Nik