As mentioned before, the file is in ~/Library/Preferences/
.
But preferences will be found in more than one file.
.GlobalPreferences.plist
com.apple.driver.AppleBluetoothMultitouch.mouse.plist
com.apple.AppleMultitouchMouse.plist
com.apple.driver.AppleHIDMouse.plist
Note that .GlobalPreferences.plist is invisible.
Mouse speed is called scaling
in .GlobalPreferences.plist
.
Setting scaling
past 3 with defaults write -g com.apple.mouse.scaling 5
has no effect immediately and may require a logout.
file://~/Library/Preferences/com.apple.AppleMultitouchMouse.plist
file://~/Library/Preferences/com.apple.driver.AppleHIDMouse.plist
file://~/Library/Preferences/com.apple.driver.AppleBluetoothMultitouch.mouse.plist