"When we made changes to the box for 4K we unlocked a little bit more processing power to enable HDR. That's it. It's exactly the same architecture as the existing one," Penello told me during a presentation on the new console. "To support HDR, we give people access to a little more CPU and GPU. That's it. It is so insignificant it is mind boggling it is being picked up."Even though Microsoft claims there's no difference, the developer of Gears of War 4 claims there is definitely a difference:
...
When asked whether there would be any performance difference, Penello was clear:
"I promise, it won't even show up. It will have no impact on games at all. Literally no impact. They will be identical. It's nothing, it is literally nothing."
Gears of War 4 employs a dynamic resolution system that tweaks the game's rendering resolution in both single-player and multiplayer to maintain its target frame rate in each mode (30 frames per second and 60 fps, respectively). According to Fergusson, the Xbox One S has additional raw GPU and CPU power compared to the Xbox One, and The Coalition's engineers have been able to take advantage of that to reduce the frequency of frame rate or resolution penalties in more demanding sections of the game.
Source: EuroGamer