When i want change speed mouse, i open settings and change it in mouse section, but this don't work, so i need change it manual, using this command "xinput set-prop 8 'libinput Accel Speed' -0.45 "
PLASMA 5.12.1
Tha mouse KCM doesn't support libinput yet, I'm afraid. It's a priority for Plasma 5.13. *** This bug has been marked as a duplicate of bug 350688 ***