https://www.gamespot.com/articles/destiny-2-dev-discusses-nightfall-changes-and-more/1100-6456672/

Bungie has shared further details on the big changes coming to Destiny 2's Nightfall Strikes. One of the biggest changes is related to the countdown timer. Game director Christopher Barrett started off by saying Nightfall Strikes should allow players to determine their own level of challenge. If players want to zip through it as fast as possible, they should be able to; and players who want to play "slow and steady" should have that option available to them as well.

Senior design lead Tyson Green said in Bungie's extended blog post that a lot of potential Strike players have passed over Nightfall Strikes because "it is both difficult and indexes performance solely on speed." Under the new setup, there will still be a timer, but your score will "bleed" over time. "We are watching this closely--score decay can feel bad, but all else being equal, a team that clears faster than another team should score higher. Score decay achieves this in the most transparent fashion," Bungie said.