Edit file XCOM Enemy Unknown\XEW\XComGame\Config\DefaultInput. Under section [Engine.PlayerInput], type ( Z-change to your own key, 1.6-change to own speed):
.Bindings=(Name="Z", Command="slomo 1.6", Alt=False)
Under section [XComGame.XComTacticalInput], change the line:
.Bindings=(Name="RightMouseButton", Command="Right_Mouse_Button_Press | onrelease Right_Mouse_Button_Release", bPrimaryBinding=True)
to :
.Bindings=(Name="RightMouseButton", Command="Right_Mouse_Button_Press | slomo 1.33 | onrelease Right_Mouse_Button_Release", bPrimaryBinding=True)