This is done on server side and this engine is very slow. If to switch Qt4 to raster engine, then performance goes to normal.
And that's why raster engine is the default since 4.8 and xrender is no longer present in 5.x. None of these improvements is present in Qt 3 btw, as Qt 3 only supports the slow XRender implementation.
Then what's the explanation for QT4 being way slower than QT3 on e.g. ('cause I have access to this hardware) X61 and GeForce GTX 470?
Cheers Martin