There's another option in the 'raster' graphics system, which does all rendering client-side in your application using (as you'd expect) Qt's software rasterizer, which is adequate enough for performance on most desktops, but still not quite optimal on desktops: hardware acceleration is the missing goodie.
Qt as of 4.8 also includes what's known as the MeeGo graphics system - as the name implies, it's used on the Nokia N9. It uses hardware acceleration (plus some additional EGL extensions) to perform absolute magic and make your pixels (especially QML :)) fly, so if you're working on getting a device together using Qt 4, I'd highly recommend you look at it.
Because no blog post is complete without a video, here's one:
(comparing the performance of raster and MeeGo graphics systems on a Lenovo S10-3t)
This sounds great, but there's one caveat. If you're on certain types of graphics hardware (SGX in particular), then you'll probably not to just want to enable the MeeGo graphics system for everything, because you'll end up with a lot of GL contexts allocated, which is not good for two reasons; they're scarce resources, and they take up a large chunk of RAM (something in the order of 5-10mb, depending on your PVR configuration file. I never actually looked).
There is good news at hand, though! Qt has *another* graphics system, specifically designed to proxy everything through another system, and allow for runtime switching back to raster. In our case, obviously, we want it to use MeeGo's graphics system by default, but fall back to raster to avoid taking extra resources when not needed.
There's also some bad news: it didn't work out of the box with Qt 4.8 RC1.
But some more good news: I fixed it (and yes, I upstreamed the patches)!
are the two commits you'll want.
After that, configure Qt with -graphicssystem runtime, and -runtimegraphicssystem meego (or set QT_DEFAULT_RUNTIME_SYSTEM=meego), and it'll work beautifully, provided your system has the required GL extensions.
Alternatively, you can use Mer, which already includes these patches right now, and, as of this week, has this beautiful magic enabled by default on the Nokia N900/N950/N9 hardware ports, where this is really needed (and works well). I'm working on getting it enabled by default on other systems that can support it, too, like the Lenovo S10-3t, as time permits.