Changing mouse wheel speed in Ubuntu

First, install imwheel via terminal:

sudo apt install imwheel

Next steps:

gedit ~/.imwheelrc and paste:

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

where you should try different values for # in the lines

None,      Up,   Button4, #
None,      Up,   Button5, #

To test the settings use the command killall imwheel && imwheel -b "4 5"

Open Startup Applications and add imwheel -b "4 5"

Note that using the option -b "4 5" restricts imwheel to only affect the scroll wheel,