This only happens when I use the trackpad. When I try to move the cursor vertically (up and down), the cursor moves very fast. When I move it horizontally, it moves at regular speeds.
My screen size is 1366x768.
Because my screen is 1.77x wider than tall, I went into /usr/share/X11/xorg.conf.d and added two lines:
Option "HorizResolution" "1"
Option "VertResolution" "1.77"
This did nothing.
Here is the edited section:
Section "InputClass"
Identifier "touchpad ignore duplicates"
MatchIsTouchpad "on"
MatchOS "Linux"
MatchDevicePath "/dev/input/mouse*"
Option "Ignore" "on"
Option "HorizResolution" "1"
Option "VertResolution" "1.77"
EndSection