first let me say that your CPU usage doesn't look that high, depending on what your configuration is. For example, if you are using 3D effects, this may be one reason for high CPU usage. Another reason may be the driver for your graphics card. I once had similar problems to yours with a different distribution. My CPU usage was constantly above the high thirties, but I had Gnome instead of KDE. It turned out that the driver for my graphics card caused some problems due to a bug.
Turning off desktop effects made the situation somewhat better but didn't solve the problem completely. I also turned off unnecessary services. I know nothing about your hardware but in my case I found it better to use less resource intensive desktops (XFCE, LXDE, etc.). However, on a different machine, the same distribution ran satisfactorily.