SUMMARY

The mouse scroll becomes slow or the steps are reduced in a way it needs several wheel rotations to scroll a few lines.
The feeling is like pedaling in cheese...

I have used KDE for a long time and I have had two behaviors depending on KDE Plasma version 5.17 vs 5.18. As the behavior changed from a KDE Plasma minor version to another, I think it is KDE Plasma-related. I have various mice, all of them Logitech and one Anker.

The scroll is slow in all KDE and non-KDE apps (Gnome or GTK based, VLC etc.), like Dolphin, VLC (volume up/down), Firefox (scrolling through pages) and so on.

STEPS TO REPRODUCE

In 5.17, in more than 50% cases, after login, the mouse scroll was slow/jumped few pixels. It was fixed by logging out and logging in again OR by unplugging and plugging the mouse or the mouse dongle. 

In 5.18.1, it seems to be related to the mouse sleep/stand by (after a while of pause in usage, due to tablet usage or simple inactivity. The scroll becomes slow and it is fixed by unplugging and plugging the mouse or the mouse dongle. 
But the login always lands with a slow scroll.
OBSERVED RESULT

EXPECTED RESULT

During usage, the mouse scroll must remain as fast as after the mouse plug in.

SOFTWARE/OS VERSIONS

Linux/KDE Plasma: Linux Sabayon (Gentoo)
KDE Plasma Version: 5.17.*/5.18.1 - now I use 5.18.1 exclusively
KDE Frameworks Version: 5.x
Qt Version: 5.x

ADDITIONAL INFORMATION


.x above means it happens over all the versions I've used in the last years, so it's a long time issue.
Why did I come now? Because I've had a simple workaround in 5.17 that lasted all the desktop session - I have long desktop sessions, but in 5.18.1 is very annoying.

I have NOT used 5.18.0 at all, so I can't tell what happens there.

This is a serious usage problem, I presume it is some KDE Plasma setup at mouse initialization.
I use Solaar, but I've had the same version over 5.17 and 5.18, with the same results and I have noticed no difference with or without it. Still, might be involved, but I can't figure out how.