How to Change Scroll Speed Programmatically with Applescript or Shell Script
Terminal:

defaults write -g com.apple.scrollwheel.scaling -float 1.0


And log-out/log-in. 1.0 is just a guess. Maybe also try 1.5. This is a hidden preference so default value is not known. Fiddle around a bit to find the speed to like. If it doesn't work, or you wish to restore to back to how it used to be:

defaults delete -g com.apple.scrollwheel.scaling