On most Symbian^3 and earlier devices Qt::WA_TranslucentBackground leads to forcing raster painting for that widget, because that is the only way semi-transparency can be achieved.
However, when there is more GPU memory available, Qt tries to use RWindowBase::SetSurfaceTransparency() to enable semi-transparency for the underlying EGL surface while keeping the openvg or opengl engine in use. This would allow having semi-transparent widgets without a performance penalty.
Unfortunately semi-transparent EGL surfaces have a limitation with current WSERV: The transparency will only work when there is another EGL surface underneath. For example Avkon content (all of which goes to the common "UI" surface) will never be visible.
This has to be solved somehow, most probably by changing Qt to always fall back to raster when WA_TranslucentBackground is present, regardless of the underlying hardware.