I'm using KDE 4.1.87 (from openSUSE 11.1 repository) and previously 4.1.3, with nVidia 180.18 and 180.22 drivers.
I've noticed on all that configurations that after using system for a while KWin consumes more and more of the CPU.
If I change the OpenGL options and turn the Use VSync on the consumption goes down to 1-2% and then slowly increase in time to 20% level.
If the Use VSync is off again, consumption is down to 1-2% and then climbs up even to 80%.

Any tips?

This might be related to this bug: http://bugs.kde.org/show_bug.cgi?id=154270
have you tried the tips at the bottom of the bug report.

The bug describes the situation pretty well, but the solution of changing something in xorg.con doesn't apply to 180.22 since all it turns them automatically.

Killall -9 kwin ; kwin

resets the CPU usage but is starts to increase it again.

I can only see the problem disappearing after turning off Kontact (while it is docked in tray the CPU consumption by KWin increases).

Could someone, please check if it is not single case?

I can confirm kwin increasing cpu after a while in 4.2 RC1 but I'm not using kontact, so obviously that is not the problem I think...

Is GLib still bothering KWin? I set QT_NO_GLIB="1" in my environment.

It must be something Kontact/KMail tray icon related since I kept the window open for very long time and there was no effect on KWin CPU consumption, later after docking it in tray I found the problem is back - I've 20% of the CPU wasted by KWin right now.

With both kontact and kmail on, I cannot reproduce it :-S I'm always at 1-2% though. I am using openGL compositing.

Maybe this bug is nVidia related?
The situation happens only after few Kontact window open/closes (exactly like described in the bug report).
If I start Kontact and keep the window open KWin stays at 1-5% CPU activity.

OK I use nVidia card (6600) with 180.22 driver (new one) and I can to some extent reproduce the bug...
When I start Kontact it does surges to 8-10% but then it falls down to none. When using desktop effects (like cube) I get 20%.
After I close the Kontact (nor can I reproduce this with KMail) (sand it to system tray) It stays on 7-5% but then it falls out of the cpu consumers list.
So I don't know if this means that I reproduced this bug or this is just normal behaviour (I think it's the second).
I can also reproduce to some extent the Xorg part of the Bug 154270, as Xorg is one of my constant consumers (but mainly it's on 1%)
And a bit off topic, I noticed that plasma also consumes 4-5% constantly.
But I don't think that I can fully reproduce the bug. And Iguess that system and drivers wise my system is quite close.
Maybe specify your hardware details.

I have:
AMD 3000+ Sempron (32bit)
NForce 4(not sure) motherboard (by Winfast)
2GB RAM
6600 GeForce (maybe GT, but I'm not sure)

My config is:
Core Duo 2 2.5 GHz processor,
2GB memory
nVidia Quadro FX570M graphics card.

Keeping system idle I've >20% KWin on CPU and second is X at 2-5%, rest varies 1-2% changing constantly.

If I won't turn the Kontact or keep it not in tray I have fairly CPU balance.
If I change the VSync (form on to off or opposite) the KWin CPU "resets" to 1-2% and keeps rising.
Having VSync on I reach 25%
if VSync is off KWin skyrockets up to 80%.

...
It must be Kontact/KMail related since I've left the system working overnight without Kontact running and in the morning the CPU consumption was still at very low value (KWin was 1-2%).
The minute after I've turned it on KWin CPU increased to 30%.

I've just updated to 4.1.96 (RC1) and the problem still remains - KWin skyrockets the CPU usage with Kontact in tray and Composite effects on.[hr]
And more information on the phenomenon:
if Konqueror is closed to tray and KWin hitting 50% CPU I can disable and enable composite effects the KWin CPU consumption bounce back to normal, 1%. Whenever I bring Kontact element from tray and send it back there - CPU consumption from KWin climbs again back to 30-50-90%.

Weird and frustrating.

It is not Bespin theme related problem either, cause the same problem happens if I chose Oxygene.

Solved !!

The bug is related to the one reported here:
http://bugs.kde.org/show_bug.cgi?id=177985

Disabling Magic Lamp and/or Minimize Effect turns everything back to normal !