Blur effect cause to low frame rate when many (>10) windows are open. For example, if open 10 "konsole" windows at same desktop the frame rate goes down from ~57fps to ~38fps. If start moving one of the windows, or if task-bar tool-tip appear - it gives ~11fps. This happens even if I move window at empty desktop area. Also blur effect causes to slow drawing with gimp (pencil more slower than mouse pointer). I get this problem at KDE 4.6 .. 4.8. With graphic cards ATI Radeon HD2600 XT and AMD Radeon HD 6670.

I expect greater frame rate with blur effect enabled. I check similar feature with other desktops:
1) Using Ubuntu 10.10 with gnome2 and compiz with gaussian blur effect enabled on my old machine with ATI Radeon HD2600 XT don't caused to such decrease of the frame rate (unfortunately, compiz blur don't work for me now, so I cant check it now).
2) Windows 7 don't have this problem nor at my old machine, nor at new.

Some screenshoots (fps at top right corner)
With blur

Without blur