Re: KDE slows down after some idling
It may be the scheduler (cfq is default) or the cpufreq control in the kernel, or both, or interactions between them. You could try deadline or noop and see if it makes any difference. In your BIOS you may have the option not to let the OS control the CPU speed -- if so you might want to try it that way. My Asus board has that.
The consequence of forcing it to run at its highest speed, of course, will be more energy consumption including when it is running and you aren't using it.
Intel Core i7-950 / Asus P6X58D-E / Nvidia GTX480 / siduction 64-bit on OCZ Revodrive SSD / KDE4.10.2/ Kubuntu 13.04