Re: Suggestion KDE 4.3 rendering speedup 190.36
XRender runs fast, but is fairly useless on LCD since it'll tear on most drawing. Any combination of OpenGL on Kwin with Indirect or vsync disabled should produce distinctly similar results, it certainly does for me.
Coolbits wouldn't affect rendering speed (obviously), on demand vblanks could only slow it down. X11 Composite wrapper and PixmapCacheSize might, but direct synchronized (needed to avoid tearing due to lack general windowed vsync equivalent) vs indirect or unsynchronized rendering is the largest difference for me.
Nvidia probably won't bother much trying to optimize for the 'touch twice' overhead (which affects particularly fast drawing things), since it doesn't affect real world 2D or 3D very significantly, and improvements to kwin and compiz are likely to produce less overhead for most common operations and cases over time, as they have in the past.
Also, slow Qt4/KDE performance can be affected due to Qt's GUI component not being compiled to use 'raster' mode by default, which significantly improves performance over X11.
Try launching the slow Qt4/KDE4 apps (but not kwin) with --graphicssystem raster command line option (requires Qt 4.5 or better) and see if that helps. It should.