For some time now, I have noticed that my laptop fan started to make a lot of noise after I locked my screen. Switching to a console and running (a)top, I found that the kwin process was causing this by using 100% of one of my four CPU cores. I found that the problem goes away when I use the Simple Locker instead of a Screen Saver.
I’m using KDE (currently 4.12.5) with X.Org 1.15 on Gentoo Linux, and my settings in KDE System Settings / Hardware / Display and Monitor / Screen Locker used to be
Screen saver / Blank Screen / Setup / Black, which I assumed would save as much energy as possible since my GPU and screen would do nothing and my screen would emit as little light as possible. Since the process causing the trouble was not kblankscrn.kss, I didn’t suspect the screen saver.
However, I found that if I use the Simple Locker instead of Blank Screen, the problem goes away. For most or all other options (other screen savers, Desktop Widgets) the issue seems to exist as well. I should probably file a bug report, but surprisingly few other people seem to have this issue (I think I found one).
On a possibly related note, I found that kwin will also start using 100% of one of my CPU cores when my screen is unlocked and I switch to a text console (e.g. by pressing Ctrl-Alt-F1).
AstroFloyd's blog 
Hey there,
have you had this bug with recent installations?
I am experiencing exactly the same but on
KDE Plasma Version: 5.18.5
KDE Frameworks Version: 5.68.0
Qt Version: 5.12.8
Kernel Version: 5.4.0-31-generic
OS Type: 64-bit
Processors: 12 × Intel® Core™ i7-9750H CPU @ 2.60GHz
Memory: 31,0 GiB of RAM
In this version there is no possibility to switch to Simple Locker
No, I’m afraid that the problem must have gone away at some update in the last six years…