I have strange problem with performance. All works fine after I've just logged in to the system. If I run only Firefox and Dolphin — scrolling works fine and all is great. But when I run a few more applications and after some time all interface becomes very slower. Scrolling in all applications begins work choppy. During idle CPU is 1-3%, but when I start scrolling (or simply moving the mouse cursor above widgets) it's grows up to 70-100%. In this time X using CPU intensive. If I close a few of applications (even if just one) — interface becomes faster but not for long time.
I have already described this problem here — http://bugs.kde.org/show_bug.cgi?id=234463