I am on Kubuntu 15.04, plasma version 5.2.2, Kernel version 3.19.0-28-generic. Whenever I do anything, I experience a huge CPU spike, and my computer runs terribly slow.

- Just writing this post, top reports 20 to 50% CPU use via firefox and 10 to 30% CPU use via Xorg.
- Opening a pdf in Okular via Dolphin causes those programs, along with Xorg, kwin_x11, plasmashell, and preload to spike 10 to 30% CPU use (Okular shows up to 50% CPU use).
- Editing a text file in kate reports up to 50% CPU use for opening it, and 30% for adding just a few lines of code.

I am unsure whether this is a KDE issue or not, but any ideas are very much appreciated.

Using the vesa driver (ie. software emulation everywhere)?
Check /var/log/Xorg.0.log

I think I am using the vesa driver. It's installed and it appears in the log.

I kept investigating the issue might be that my CPU governor is stuck in powersave. Even when I change it to performance the frequency is stuck the minimum (20% normal frequency). /sys/devices/system/cpu/intel_pstate/{min,max}_perf_pct are both 100, and I changed /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor to performance. Still no improvement.

This seems to be an issue with intel's scaling driver `intel_pstate` and independent of KDE.

In case anyone encounters the issue, going back to `acpi-cpufreq` sort of fixed it for me. I added `intel_pstate=disable` to `GRUB_CMDLINE_LINUX_DEFAULT` in `/etc/default/grub` and ran `sudo update-grub`. On reboot things were OK.