Honestly, it should be easier – then again, it’s Gnome, nothing really is easy.

Collecting a combination of various pages (this one finally yielded the right directions) it turns out one has to install imwheel and then adjust the ~/.imwheelrc file:

# .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

The magic incantation is in the first couple of lines and the value (3 in the example) is what will change the scroll size: in my experience, the baseline is 3 lines, and the actual value is multiplied by the factor given here (in our example, every tick on the mouse wheel will cause a 9 lines’ scroll).

Finally, for the changes to take effect:

$ imwheel -kill

no need to sudo or logout.

Really cannot for the life of me understand why Canonical cannot add that control alongside the mouse speed setting in the System Settings.