As mentioned, Cairo is probably the main user, which is built into a lot of GTK apps. Qt using the native drawing backend uses it as well. However, the compositors and desktops themselves are basically OpenGL based and don't use it at all. It's the apps themselves which use it.In which use cases is it really used if one uses say KDE, GNOME3, XFCE or Unity with compositing turned on?
Are you saying it is only used for things like LXDE / compositing turned of?
With Wayland, XRender will likely largely go away. XWayland apps will still use it, but native apps will probably just use standard OpenGL acceleration.
I'd be more concerned about bugs than performance at this point.
This is coming from someone who is NOT familiar with the layout and workings of TrueAudio and similar dedicated audio blocks, so if its not in the same design or style as video decoding blocks then just ignore me haha. The idea of it was just abstracting away the platform details so that we can just say "Oh you know you want dedicated video decoding? Just target VDPAU. Oh, YOU want dedicated audio handling? Target $Insert_Wrapper_Name"