Does IS help with slow moving subjects?

It would always help reduce vibration to a certain extent. Depends how many stops the IS is. 4 stops pretty good. Any higher than that is excellent. In low light, IS might indirectly be handy since it allows for slower shutter speed so more light can be gathered. If you are shooting people walking, 1/60 shutter or faster should be good enough with IS. Without IS, I recommend you go at least 1/100.