yes r5xx ... which doesn't seem to be getting any love from anybody, really ... i'm stuck in the middle, with what seems to have been the last incarnation of a pretty old architecture ... i feel robbed from not being able to use a driver that's able to tap into the full range of feature my card is capable of ..
Why do you say that? r300g is still seeing regular updates and improvements. It would also help if the kwin developers actually tested their code on non-nvidia hardware and filed bugs for problems rather than just blaming the drivers, but I digress...
pedepy, are you pretty confident that you are really running the 300g driver ? You mentioned it in conjunction with the radeon ddx driver rather than Mesa, so just wanted to confirm, since r300g is a build option for mesa, not the ddx.
Can you post the output of glxinfo | grep render just to be sure ?
I've tried kwin 4.4.2 (kwin --replace) and discovered that it uses indirect rendering i.e. it detected GL1.4 (2.1 ...). Why? If compiz can use direct rendering, why can't kwin?
Older versions of KWin (<4.5) used to force indirect rendering. I forgot the main reason, but it might have to do with driver issues.
With 4.5, they removed this, and used direct rendering by default, and that's when the whole breakage started (blur, crashes when applying settings, etc.), you know the story. The workaround was to force indirect rendering, until the bugs in Mesa were fixed.
In any case, KWin is not the fastest compositing 3d manager out there. People who really need top performance should continue to use Compiz with KDE, like they do with Gnome. It works just fine. For me, KWin works just fine (but I'm not experiencing the problems the OP is, the last KWin issue was about a month ago for me).
I think the ATI open source r300/r600 driver developers have focused on 3D games like openarena, but haven't paid attention to other non-3D opengl applications. Not just kwin suffers horrible performance. On my AMD 760G integrated chipset, XBMC also suffers greatly. The XBMC built-in software video renderer is much (twice) faster than its advanced shader (GLS ) or basic shader (ARB?) video renderers. It's pretty frustrating.