In a situation that is both completely unbelievable and at the same time completely expected for Linux, you can't easily set the scroll rate of the mouse wheel.
Here's the stupid hack to fix it. From here.
$ sudo apt install imwheel
Jam this into ~/.imwheelrc:
".*" None, Up, Button4, 3 None, Down, Button5, 3 Control_L, Up, Control_L|Button4 Control_L, Down, Control_L|Button5 Shift_L, Up, Shift_L|Button4 Shift_L, Down, Shift_L|Button5
$ imwheel -k
(The -k kills any old instances – re-run this if you tweak the config file.)
In your desktop settings editor, add imwheel as an autostart application.
Trivia: If you'd like to know more AND be frustrated by the nerd disease that keeps this crap broken, see this thread:
In it, a user asks a completely reasonable question: "How can I change the scroll rate?" In response, a KDE contributor spews a bunch of technobabble bullshit about Libinput, Weyland, X11, KWin, and xf86-input-libinput, and how they (KDE people) don't 'own' other modules.
In this developer didn't suffer from Open Source Blindness Disorder, they would stop and realize (1) this is an end user and (2) the fact that the organizational structure surrounding this software precludes solving this issue damns it from the start and this dysfunction must be remedied if open source software is ever going to hope to penetrate the consumer market.
The user gives the best possible response: "You gave the technical reason, but it's not helpful and as a user I don't care. The feature can be apparently be enabled with other software, which is what is wanted."
I hope the developer learns a lesson from this interaction, but I know they won't.