I've noticed that large-challenge scenes often drag on, while short ones lack impact.

What if: at scene start, players each have a card limit of 2. When the Narrator Continues, that goes up by one.

If a scene is really dragging (because e.g. some players feel like their character can't contribute, or a player is having time-availability issues), the Narrator could continue (possibly dropping an Asset or two) and let the other players wrap up the scene in a satisfying way, rather than having to end the scene unfinished.