Using a Registry hack you can change the speed the wheel zooms in and out.
WARNING: Beware of editing the regsitry it can cause system instalbility or stop you system working altogehter.
1. Windows key + "R" to open the run dialogue
2. In the Open field enter "regedit" (without parenthesis) and click OK
3. Browse to this Registry Key HKEY_CURRENT_USER\Software\Autodesk\Inventor\RegistryVersion##.0\System\Preferences\Display (where RegsitryVersion19.0 = 2015 version)
4. On the right locate "Wheel Zoom Speed", double click it and change the value as desired. the default value is "1" a value of "0.5" would be half the speed, conversley a value of "2" would be twice the speed.
4. Restart Inventor