The word imperceptibly is used for things that are hardly happening at all or happening to such a small degree that you barely notice.
If something is imperceptibly moving, then it's barely moving — or not moving enough for you to really see it moving. It's imperceptible, or not perceptible. An imperceptibly whining dog is hardly whining at all. An imperceptibly flapping flag is nearly still. This word is the opposite of dramatically or extremely, when there's a lot of activity.