Well let's say that something is happening at a certain pace. Then it starts slowing down. The reason we can say it is "slowing down" is because we have the notion of time that exists regardless of how fast things are changing, or not.

So if nothing is happening I would think it means that things are happening at zero speed. Again we can say the speed is zero, because we have the notion of time that doesn't change juts because things slow down.