Natasha is a monster.
The trick is that the slow only last for as long as their being shot. Once their out of the spray the slow is gone it has no life duration. So you just gotta get used to leading your target, which most heavies do anyway. Once you have someone in your spray their pretty much dead, running away isn't really an option unless there is a corner right there or they get really lucky. And I'm not 100% sure but I think the slow effect gets stronger the longer their in it. If you just glance a person from across the map you can't even notice but hold on a person for a few seconds, and if their still alive, they just start crawling around.