Hi
I have installed Manjaro KDE Plasma, but the system is very slow. To open dolphin file manager takes 30-40 seconds.
So I have downloaded the latest Manjaro KDE plasma ISO file and used a different USB drive to make bootable drive. I can tell
just from running the usb live session that it's very slow.
I decided to try different distro and I picked Solus KDE & Fedora KDE It's the same from live session, very slow.

I have tried Kubuntu and that seems to be ok. I haven't install it but from live session it's very responsive.

The question is, why Solus and Manjaro KDE plasma is painfully slow and Kubuntu is fine????

Any suggestions??

There are plenty of possible reasons, the first that comes to mind is your graphic card driver, the amount of RAM, the amount of free space on your disk, etc.

You definitely need to give more information about your system

I have been experiencing this behavior as well on Gentoo linux with plasma latest stable 5.21.5. It appears that while I was typing search string in google chrome there was like 1-5 seconds latency before text appeared and browsing was deadly slow. In text editors or some more lightweight applications experience was faster, but still remarkable slow down. I also use sddm with display-manager package.

Once per time I use zoom for calls, after call finished I close it, but process remain and while I am on 32 thread cpu system i noticed there is almost 100% ocupation on sddm and zoom process was eating about 20%. I killed the zoom process and all went right faster. It seems like sddm only works in single threaded way and if somehting takes the single pipe, all other apps suffer.

Check your processes if something is not eating your display manager time.