It is said that they slow down the rebel fleet, but how does that works exactly? Is the fleet slowed down when they encounter a nebula beacon? Or is it when the player explore a nebula beacon? Or do you just need to have nebulae beacons in a system to make the fleet slower?
When the player "explores"/visits a nebula beacon, the rebel fleet advance for that turn is cut by half. i.e. if you visited 2 nebula beacons continuously, the rebel fleet would advance as much as it would have if you had visited 1 normal/non-nebula beacon.
In Uncharted Nebula sectors the enemy fleet will be prepared for the nebula. This means that all the nebula beacons in the sector will only slow them down to 3/4th of their original speed. So even though the sector is full of Nebula beacons you would not gain double the amount of turns as a normal sector.