Comment 32 for bug 825548

it seems that something scales the touchpad x/y speeds to the virtual screen size, without respecting aspect ratio. as this does not happen with other mice, the synaptics driver probably is to be blamed.

there's a crude hack for this bug: mimick the aspect ratio via setting the virtual screen size with xrandr

1) figure out the virtual screen size (the screen size is automatically shrink-wrapped to the smallest possible size to contain all monitors)
e.g. on my laptop, with an external monitor connected:

~$ xrandr
Screen 0: minimum 320 x 200, current 3360 x 1080, maximum 8192 x 8192
LVDS1 connected 1440x900+0+180 (normal left inverted right x axis y axis) 261mm x 163mm
   1440x900 50.0*+
   1360x768 59.8 60.0
   1152x864 60.0
   1024x768 60.0
   800x600 60.3 56.2
   640x480 59.9
VGA1 connected 1920x1080+1440+0 (normal left inverted right x axis y axis) 509mm x 286mm
   1920x1080 60.0*+
   1280x1024 75.0 60.0
   1152x864 75.0
   1024x768 75.1 60.0
   800x600 75.0 60.3
   640x480 75.0 60.0
   720x400 70.1
HDMI1 disconnected (normal left inverted right x axis y axis)
DP1 disconnected (normal left inverted right x axis y axis)

2) increase the screen size to have the same aspect ratio as your synaptics touchpad (here, it's ~3360/1.938):
~$ xrandr --fb 3660x1734