As of today, Microsoft has enabled FPS Boost for five popular titles on the new Xbox Series X and S consoles – Far Cry 4, New Super Lucky’s Tale, Sniper Elite 4, UFC 4, and Watch Dogs 2.
FPS Boost will do what its name suggests: increase frames per second two to four times. This means that a game like UFC 4 can now run at 60 fps, while New Super Lucky’s Tale can run at 120 fps.
What’s most impressive is that the FPS Boost feature doesn’t require any game updating or any work from the game developer. In practice, the game thinks it’s still running at its old frame rate, while FPS Boost does its magic to provide a higher one. You can find a detailed explanation of FPS Boost by Digital foundry in the video at the end of the article.

The Xbox team will bring a new menu to supported games called Compatibility Options. There, players will be able to turn FPS Boost (and Auto HDR) on and off.
Source | Via

