"Descent Into Madness" atlas passive node, delirium increases faster

"Delirium in your Maps increases 50% faster with distance from the mirror"

What does this mean exactly? Will the speed (of the mist expansion) be uneven? (Bigger distance - bigger speed?) Will the encounter end sooner?

What is the benefit? (Some characters run to the opposite end of the area and are waiting the mist to arrive?...)

When you do a mirror, the further from it you are, the higher the delirious value (and therefore the tougher the stuff and the more rewards). That node makes that counter run up faster.