KDE 4.9 will be released in August and overall it feels more responsive and faster during desktop use than its predecessors.
Aside from Qt Quick support in KDE Plasma Workspaces, improvements to the Dolphin file manager, and other changes, performance was also a focus for this six-month update to the KDE desktop. There's many performance improvements and bug-fixes to improve the user experience.
I've heard good comments about speed improvements of the Plasma desktop within KDE SC 4.9, so I tried it out for the past few days -- comparing Ubuntu 12.04 with KDE 4.8 to the latest KDE 4.8.90 packages. Yes, indeed, KDE 4.9 feels much nicer!
I was also curious to see whether KWin compositing window manager improvements for KDE 4.9 had any benefit to running OpenGL full-screen games. It, however, doesn't appear to be the case at least when using Intel Ivy Bridge graphics with the Mesa 8.0 DRI driver. When using the default settings (i.e. not un-redirecting full-screen windows), there wasn't any major change in performance.
System details, system logs, and more OpenGL benchmarks from the KDE 4.9 development desktop can be found at OpenBenchmarking.org. You can run your own comparative benchmarks via phoronix-test-suite benchmark 1207187-SU-INTELIVYK22 with the Phoronix Test Suite open-source software.