Ubuntu 18: Set usb mouse speed / sensitivity (panel and xinput not working)

I had the same issue. I was able to fix it by changing the "Accel Speed" property in xinput. It didn't occur to me at first that it's possible to set negative values for it, but when I tried it I was happily surprised my mouse is now going slower again.

So full info, first type "xinput list" to get a list of devices, your mouse should be in the list:

~$ xinput list
⎡ Virtual core pointer                      id=2    [master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer                id=4    [slave  pointer  (2)]
⎜   ↳ Genius Optical Mouse                      id=8    [slave  pointer  (2)]
⎣ Virtual core keyboard                     id=3    [master keyboard (2)]
    ↳ Virtual core XTEST keyboard               id=5    [slave  keyboard (3)]
    ↳ Power Button                              id=6    [slave  keyboard (3)]
    ↳ Power Button                              id=7    [slave  keyboard (3)]
    ↳   USB Keyboard                            id=9    [slave  keyboard (3)]
    ↳   USB Keyboard                            id=10   [slave  keyboard (3)]

In my output pasted above, my mouse has ID 8. We can use this to find the mouse properties:

~$ xinput list-props 8
Device 'Genius Optical Mouse':
    Device Enabled (148):   1
    <...snip...>
    libinput Accel Speed (294): 0.000000
    libinput Accel Speed Default (295): 0.000000
    <...snip...>

We find the property number for Accel Speed (294 in my case). We can use this to lower it below zero:

xinput set-prop 8 294 -0.5

Note you need to change the 8 with your mouse ID, and the 294 with your "Accel Speed" property number. I found -0.5 to be an acceptable value for me, but you might want to set it to something different.

To make this change permanent after reboot, you will need to run that command automatically at startup. I'm not sure how that works in vanilla Ubuntu, but with XFCE (Xubuntu) you can do "Settings -> Session and Startup -> Application Autostart -> Add" and then fill in the full xinput command as command, and a nice name and description.