Never call QPainter::setOpacity() when painting on a QPixmap or a QWidget. QPainter::setOpacity() effectively disables all hardware acceleration, causing all subsequent rendering operations to be done in software. This means that a call to QPainter::setOpacity() in a performance critical path is the kiss of death.
An alternative approach to the problem
Create a temporary pixmap, initialize it to fully transparent, and then draw on it with normal opacity. When you're done drawing on it, use the DestinationIn composition mode to reduce the alpha of the pixmap, and then draw the pixmap itself where you want the semi-transparent contents. If you need the same content again (for example when you're doing a fade out or fade in animation), it might pay off to cache the fully opaque version.
// Do the rendering with full opacity
p.fillRect(pixmap.rect(), QColor(0, 0, 0, opacity));
painter->drawPixmap(0, 0, pixmap);
Note that you can check if setOpacity() is fast by calling painter->paintEngine()->hasFeature(QPaintEngine::ConstantOpacity), but my recommendation is to avoid it completely since it won't be fast on our main platform with the current Qt version (4.4 beta).
Never use QPixmap::setAlphaChannel() to make a pixmap semi-transparent. In fact you should never ever call QPixmap::setAlphaChannel() for any reason if you can avoid it. This function will convert both pixmaps to images (this involves two blocking synchronous calls to the X server, plus expensive image transport over the socket for each call), expensive format conversion of both images, copying the alpha channel one pixel at a time from the alpha image to the other image, followed by a final conversion of the resulting image back to a pixmap (which is also a very expensive operation).
The right way to do it
Check if the pixmap has an alpha channel, using QPixmap::hasAlphaChannel(). If it has one, start a QPainter on the pixmap, and reduce the alpha using the DestinationIn composition mode. If the pixmap doesn't have an alpha channel, you're going to have to create a new pixmap that has one and copy the content of the original pixmap into it, like this:
p.drawPixmap(0, 0, pixmap);
p.fillRect(temp.rect(), QColor(0, 0, 0, opacity));
pixmap = temp;