Hey guys,
I'm on KDE Neon 5.12.4 and I'm having an annoying issue : The system freezes quite often, especially when I don't touch anything for a certain while (watching a vidéo, leaving the system on at night, etc). Sometimes I got a black screen with only the cursor on it, others it just freezes. The only way to get out of it is to hard restart to computer. Notice that screensaver and screenlocker are disabled.
I'm on an Asus X541S laptop, CPU : Intel Pentium N3710 QuadCore, GPU : Intel HD Graphics 405, RAM : 4 GB.
Thank you for helping me out.

Slideshow wallpaper?

@Zren : Yes. Could it be the cause ?

Definitely https://bugs.kde.org/show_bug.cgi?id=368838

There's been a long standing bug that's only been recently fixed in Qt. Since you're probably not using an unreleased version of Qt, slideshow will be unstable for now.
It's possible that the slideshow might be usable when Plasma 5.13 is released, but you might have to wait for the Qt 5.10 upgrade.
For the meantime, you'll have to do without the slideshow and use a static picture.

@Zren : Thanks a lot

I have also experienced similar sluggishness recently. Didn't realise that it started after I chose slideshow for desktop.

Thanks Zren.

A nice post, thanks in advance.