Tip: Performance/smoothness with nVidia cards: Smoothness of KWin rendering can be improved by setting the env.variable KWIN_NVIDIA_HACK to 1 (e.g. append ‘export KWIN_NVIDIA_HACK=1’ to your ~/.profile file). This sets ‘__GL_YIELD=NOTHING’ for KWin, letting KWin use more CPU time for OpenGL operations, however at the expense of affecting performance of other applications. This option is enabled by default since KDE4.0.3, as the performance impact seems to be non-significant. See section ‘OPENGL YIELD BEHAVIOR’ in README.txt for nVidia cards for details.
Tip: In some cases, overall smoothness may be increased by turning off direct rendering in advanced options in the Desktop Effects configuration module (Alt+F3->Configure Window Behavior).
Tip: Changing the Texture filter in the Advanced Compositing Options to Nearest may improve performance considerably (at the cost of aliasing artifacts), especially on low-end graphics equipment.
For the first tip just open .profile file in your home directory and type “export KWIN_NVIDIA_HACK=1” without quotes.