Well, this topic is not a question, but rather how i've found out the solution to plasmashell high CPU usage, under a certain situation.
Problem description:

--- You can skip this little history if you want ---
After the **** Bugsoft Windows 10 has broken my EFI partition due to another update (it was a dualboot setup), i decided to (finally!) get rid of this ****. I reinstalled Manjaro linux and the EFI boot partition was recovered, so i started tinkering with VirtualBox and installed a Windows 7 virtual machine. Everything was fine until i decided to uninstall VirtualBox because it was very slow even with a killer fast gaming machine, Hyper-V acceleration, etc... then problems started to appear!
--- End of history ---

After uninstalling the following packages
{
linux414-virtualbox-guest-modules;
linux414-virtualbox-host-modules;
virtualbox;
virtualbox-ext-vnc;
virtualbox-guest-dkms;
virtualbox-guest-iso;
virtualbox-guest-utils;
virtualbox-host-dkms;
}
and restarting my system, i've noticed a noticeable overall slowdown on my computer. Imediatly i opened Ksysguard and checked CPU usage: plasmashell and kwin_x11 were spiking at ~30% cpu usage on all the 4 cores, every one or two seconds. I'm not anything near an experienced Linux user, but after lots of searching on Google, i came up with some solutions to other similar problems, like deleting ~/.config/plasmashellrc and ~/.config/plasma-org.kde.plasma.desktop-appletsrc, killing all plasma related services and then restarting, changing the Noto Sans system font... none of then worked!

Solution:
After some (long!) time, i did my normal system maintenance routine and executed this command on the console:
sudo pacman -Rs $(pacman -Qqtd)
I think you all know this, but anyways... all it does is to delete orphaned packages from the system. The following packages were removed:
{
dkms-2.5-3
gtest-1.8.1-3
net-snmp-5.7.3-9
zenity-3.30.0-1
}
and WOOSH! CPU usage dropped to around ~2% per core like magic, the same it was after a fresh install of Manjaro.
So how the **** this worked?! Maybe one of the packages was conflicting with plasmashell? I don't know, it just worked.

I hope this topic really helps someone else out there, because i signed up to this forum just to share my experience with others!
Sorry for my BAD english, i really tried hard...