In a series of tweets, Destiny 2 Project Lead Mark Noseworthy confirmed that due to the game being CPU bound the game will only run at 30fps. Noseworthy also noted that even if you lowered the resolution from 4K to 1080p you would still not be able to run the game at 60fps.
Noseworthy finally revealed that they will be able to evaluate 4K resolution once the once the hardware is released. This is in part due to the game coming out in September, but the Xbox One X not being released until November. Stating..
” We would never hold back game performance on a platform to appease a partner. No partners asked us to either. We optimize for each platform’s tech to deliver the best social action game experience we can. Period. All consoles will run at 30fps to deliver D2’s AI counts, environment sizes, and # of players. They are all CPU-bound. We are currently evaluating 4K for Xbox One X. It launches after D2 and so we are focused on the launch consoles right now.”