In Zack's blog post, entitled 2D in KDE, he describes at length these three graphics rendering engines for Qt. Obviously, using the CPU raster engine is the most taxing while using X Render is better, but using OpenGL is the best, generally. Zack concludes his lengthy blog post with "the quicker we get the rendering stack to work on top of OpenGL the better off we'll be."
Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.