SUMMARY I would like there to be an option in System Settings under Mouse Settings to change mouse scrolling speed. If you compare this to the settings in Windows, this is one of the only mouse settings that is missing and it is a shame. Many people find the scrolling speed too slow and would like to change it using a nice slider. It is very important for customization. I think the most user-friendly unit would be lines as opposed to some decimal value, but that's on you.
On Wayland there is an option to change scrolling speed. For X11 see https://bugs.kde.org/show_bug.cgi?id=448429