Depending upon the driver it can potentially cause a hit as shown in Wednesday's comparison of Unity, Unity 2D, GNOME Shell, KDE, Xfce, LXDE, and Openbox. All of the desktop environments were tested in their "out of the box" / stock configurations on Ubuntu 12.04. The KDE aspect is being discussed in this forum thread where the usual items are brought up.
There's interest in more KDE KWin directed vs. undirected fullscreen OpenGL rendering performance, so that will come in a separate article looking specifically at that item rather than mixing default and non-default test parameters.
However, in the meantime until the next set of results are complete, an independent Phoronix Test Suite user has conducted his own KDE graphics benchmark results and shared them on OpenBenchmarking.org.
Hit up 1204109-BY-KDE3DDESK93 for a number of gaming (Nexuiz) results under KDE with 3D on and off. Testing was under Fedora 16 with an AMD E-350 Fusion system and its Radeon HD 6310 graphics. The individual also used the Phoronix Test Suite's system monitoring module to collect various system vitals while Nexuiz was being run.
More Phoronix.com benchmarks are on the way, but until then there's other Phoronix Test Suite users sharing their data on OpenBenchmarking.org and taking advantage of new features.