Hmm, I've just read of a little bit opposite. On Xorg Qt4 uses XRender for composing by default. This is done on server side and this engine is very slow. If to switch Qt4 to raster engine, then performance goes to normal. Here is an article:
"How to drastically boost up your KDE 4 performance by using the Qt raster rendering engine"
To put it simple: it does not work.