I'm doing some research on performance of a Qt App on Linux and I'm not an X11 person...what is the effect of turning on and off the QT_NO_XRENDER flag?
At a guess I would say defining that variable prevents Qt from using the XRender extension. ;-) The XRender extension allows certain graphic operations to be accelerated. However, actual driver support for XRender varies and often XRender still hits code paths that are not optimised/accelerated by hardware. I think that this was one of the reasons why Qt has concentrated on the raster graphicssystem as opposed to the X11 one recently.
To try the raster graphics system, you can specify an option on the command line like this:
@./myapp -graphicssystem raster@
This option can also be compiled into Qt as the default or enforced by the application itself via a call to QApplication::setGraphicsSystem( "raster" ).